I think the issue is that sometimes one sin can lead to a greater sin. For example, someone who sins by intentionally breaking the speed limit, could end up killing innocent people. He didn’t intend to kill anyone, but he did intend to sin, and the wages of sin are death.
A woman who dresses immodestly, knowing full well that its a sin could trigger a man’s pornography addiction. Then, that man could end up viewing such intense pornography that he becomes so stimulated that he feels the need to act it out, and ends up raping an innocent woman. Obviously, the initial woman who dressed immodestly never intended her actions to ultimately lead to that, but she did intend to sin, and as a result, another woman got raped.
As for the actual topic, bullying, it is always wrong, and always selfish, even though the bullies really have nothing to gain from it. Yes, words are powerful and hurtful, especially when the recipient feels that there may be some truth to them. Every unguarded word has the potential to lead to someone’s death, whether suicide, or provoking the person to murder innocent people, as was the case in Columbine, or other school shootings. The bullies weren’t the ones who got killed, but innocent people.
So the real issue is not so much technical “culpability” in the strictest sense, but just that we need to understand that our sinful actions can lead to a chain reaction of consequences that are far more extreme than what we had intially thought could possibly ever occur. When God gives us commandments, they are for our own good, and the good of all others. God sees the big picture and knows how the most venial of sins can lead to the most extreme negative consequences. We need to HAVE FAITH and trust in God. When we say, “I believe in Jesus,” that means that we also need to believe that EVERYTHING that He has commanded us to do or refrain from doing is said for our own ultimate good, even when it doesn’t seem that way at the time, or when we don’t understand it. We can NOT take the attitude of, “I’m willing to transgress this commandment, as long as I’m not culpable if it leads to something really bad.” Instead, we need to take the attitude of “Jesus, I trust in You,” even when we don’t understand the reasoning behind the commandments.
I realize that my reply was somewhat off-topic, but I saw a bigger issue that I felt needed to be addressed. As for the actual technical culpability in that specific situation, whether in God’s Eyes, or the eyes of the law, I don’t know. What I can say is that if someone bullies someone else, the bullied person commits suicide, and/or murders others, and the bully doesn’t feel an ounce of guilt or personal responsiblilty, then that bully’s heart is far from where it should be.
There is a secular song called “The Freshmen” by a group called The Verve Pipe, that addresses this issue regarding an abortion and a suicide, and the following guilt that ensued, how they never believed that they would, “die for these sins.”(even though no specific sin is mentioned that lead to the suicide, merely a break-up) So be careful, for whenever we transgress one of God’s commandments, well, the wages of sin is death.
Granted, I’m likely taking that “The wages of sin is death” quote out of context, but I feel that it still can hold true in the way that I am presenting it.
To the OP, realize that you have the option of the Sacrament of Reconciliation, if this is not merely a hypothetical. If so, I would explain the situation to the priest, and he will likely tell you that you are NOT guilty of the sin of murder, but merely guilty of the sin of teasing or bullying, but that you will be forgiven of that sin, after the confession.