5 year old behavior problems

Peers can influence other children as well as television. Take him to the counselor or to a psychologist to see if you can get him in cognitive behavioral therapy.

Bullying from other students? Maybe cutting his hair didn't "fix" that.

Sounds like he is having a hard time. I know it can be hard to handle those (the world is ending) reactions.
Brief description: CBT is therapy that addresses the thoughts/beliefs behind maladaptive feelings & behaviors. Not sure that I'd go looking for it for a child, but that's what it is. (It looks at the beliefs & assumptions behind our distressing feelings, with the goal of changing our feelings based on that examination & assessment.)

My thought would be to look at the responses you make to these upsetting things--his complaints about school & statements that nobody likes him, his expressions of frustration/anger & unhappiness (I hate you/I want you to die-type things), his extreme reaction to hearing No. These are not easy things to respond to, but our responses can complicate things for our kids.
I try to practice acceptance, rather than engaging the child on the level of validity (trying to talk him out of his upset.)

I know that hearing "None of the kids like me" is probably distressing to you. I don't know how you respond when he says these things, and I don't believe there's a "right" thing to say (in the sense that this situation needs fixing with the "right" thing), but that being able to care & to hear him (this is a way of accepting his feelings as they are, and giving them validity) is more important than some kind of "right thing" to say.
Our children are their own people & their feelings originate in them, and the "solutions" (for lack of a better word) do, too. This doesn't mean hands-off, sink or swim! It just means we can't do it for them. The more aware we are of this, the better for them. What they need is our support & our presence, but not "support" in the sense of fixing them/cheering them up, doing it for them, sparing them from pain, etc.
When we trust them & can be with them, we give them something great. It is hard to

see your child in pain & not be able to make it better immediately. This fact can make it hard for us to accept their pain & their perspective or beliefs/thoughts (self-doubts, negative self-judgments, etc.) We can rush in to reassure them or "correct" their mistakes, assuring them that they DO have friends or people DO like them, or that the kids aren't worth being upset about, but we are negating their messages of pain when we do this. We're basically showing them that what they fear is so intolerable or unthinkable that we can't tolerate the possibility of it being true or they can't possibly survive it themselves; we have to deny it. Cheering them up, distracting them, reasoning with them or trying to persuade them all reinforces a victim mentality, for this reason.
My advice would be to try to hear him & reflect back what he's saying--let him have his feelings & share them with him.
Are there actual behavioral

problems accompanying these feelings & upsets?
I know you mentioned aggression...how is he acting this out? Is it primarily through those angry comments (I hate you/I wish you would die) or is he acting out physically as well?
Physical anger when he's frustrated would be normal, as well. The bottom line is that it's important to keep everyone safe (including protecting HIM from hurting a younger sibling or you, because that's painful for him & damaging to his self esteem as well, to experience himself as "bad" or "mean.") In addition, a constructive response that recognizes the feelings behind his aggressive action & validates these, then offers a positive/acceptable alternative once you've connected by validating his feelings & understanding the reasons for his behavior, will help to address the behavior.

Overall, responding optimally to his negative feelings (words AND actions) will most help to relieve the "acting out."

Hi again. I just came back to comment about reflecting feelings. Before I was mostly responding to your second comments about what's been going on with him. But it applies to the situation you mentioned in your first post, too. We can best reflect for our kids when we "hear" the feelings inside their statements more than the specific content of the statements themselves. Or when we can focus on the feelings expressed rather than on the expression. This is true for all sorts of situations--rudeness, defiance/opposition, aggressive statements. Hearing the words, realizing the feelings they express, and responding to the feelings behind them (in contrast to addressing the expression itself), is what I'm talking about.
In the situation you describe, I think a 5yo who moves from loving statements to "I want you to die/I want you to be killed" or "I wish you were dead!" exclamations is expressing miserable feelings. He's showing his

sadness & his misery. Those are lonely, isolating experiences so the greatest response a parent can offer is contact--seeing/realizing & being able to understand. This is when "hearing" the misery & being able to sit with it comes in.
Children possess a limited perspective. They know that certain words & phrases are "big" or "bad," "horrible," but they don't grasp the full awareness of meaning or why. A very young child declaring "I wish I was DEAD!" or "I want to die!" or "I feel like killing myself!" is concerning because this expresses distress, very big feelings of upset that need attention, but it is not necessarily the content of the expression that is really the subject. (When questioned, the same child will tend to be very unclear about what it actually means, saying maybe he'd ride his bike really fast & try to fall off, etc.) The feelings are the important part.
It's like when an adult says "I'll always love you" or

"I will never leave you!" These declarations may be made within a commitment that DOES hold up through time, but they don't really express promises or contracts or the commitment itself. They are expressions of how intensely that adult feels about his/her love right in that moment of speaking. It would be more "accurate" for a person to say "I'm so happy!" or "I love you SOOOO much!" or "I feel so good right now," but humans tend to reach for superlatives & expansive declarations when they FEEL those things, so we get "I've never loved anyone this way before!" or "I will always love you", instead.
It can help if you think of your son's emotional expressions in this way. Notice the words but "hear" the feelings.
My guess is that things relating back to October & before are mixed into this kind of miserableness. You don't have to "know" for sure; just start listening. Being with the overwhelming sad feelings & anger.