As of this moment I can only think of two reasons why one would choose to kill themselves:
1) Out of the notion that one would be exacting revenge upon another by destroying one's self. The motivating factor of offing yourself would be that this other party, who perhaps has hurt the suicidal mongrel so, shall suffer the most extraordinary amount of guilt when the suicide has been delivered. And this great amount of guilt will drive THIS person to their untimely demise too. This is called selfishness. The thought of driving someone into despair on one's behalf (which would be death) excites them so much that they would be willing to sacrifice existing and enjoying the sweet fruits of life and perhaps witnessing this other party's despair. Kill yourself over an expectation. How lovely.
2) Killing yourself simply to rid yourself of this world. The world disgusts you so much that you'd rather not be a part of it. It would be an independent action, completely out of your own consideration and not of others. It is not your parents' fault that they've spawned you and left life to spurn you. You do not have any motivating grudge against anyone in particular. You just want everything to cease to exist. And you deteriorate so far into rags, as you can imagine Roskolnikov in Crime and Punishment, rejecting the world, feeling revulsion at even touching and being a part of it. Thus you decide simply to eject yourself from it. As simple as that.
Now think of this: theoretically, you decide to kill someone out of boredom. You've made your peace with your life, that there is no hope that you would ever become the person that your parents have envisioned, and you simply don't care. You are bored and you don't feel like slowly letting cancer kill you as you continue smoking two packs of cigarettes a day. You are too lazy to get a job just so you could afford meagre living for yourself, in an apartment, on a couch bought from IKEA, staring blankly at the television screen before drinking a glass of wine before retiring for the night. Wanking off in bed while thinking of tight bodies.
So why not kill someone out of boredom?
You are willing to accept the consequences for the action you are about to take. Society's rules dictates that when one takes another's life in a brutal manner, that one should be punished for their actions, as the law deems fit.
So, you're going to die anyway, so why not? You don't believe in religion, you don't believe that there is anything beyond living, that you will somehow be transported into to a magical place of purity where your soul will rest forever and ever in peace. You think that's total bullshit and you don't give any more thought to it.
You will live in your cell in modest luxury; you will be fed, clothed, and bathed. Sure, you might risk being assraped by some other cellmate, but you understand that this is part of your consequence.
And there will be the court trial when those related to the person of whom you have killed will try to make you feel guilty. They will try to make you feel emotional pain and regret over what you have done; taking away a life that didn't deserve to be taken away.
They might even say:
I wonder who you think you are
You damn well think you're God or something
God give life, God taketh it away, not you
I think you are the Devil itself
But you know that even if you felt guilt, even if you started to believe that what you did was wrong, soon it won't matter. You wouldn't exist anymore. That all of this might as well be seen as filler until your consciousness blinks out forever. And you accept it. Before you decided to kill someone, you accept that this is going to happen: that they will try to make you feel guilt.
And then there will be that last meal to look forward to. You can eat whatever you want. The final gorging of the belly and an attack of bliss of the senses before you are zapped away.
And you accept this is a pretty full life, it's as much as you can aspire to, because in the end, as cliched as it is, it doesn't matter at all.
But hey, this opine doesn't matter at all either, right? In this world, in this dimension, it's all irrelevent. There are no set of morals that others inhabit and keep dear to their hearts, and killing out of boredom is a common occurence every day.