I hope you have never and will never entered depression phase where you can no longer think about anyone, not even to care about yourself. Because if you have, you will probably never said those things and you will understand how it feels to unable to do anything, even something as simple as eating or taking a bath, not to mention thinking about other people.
He wasn’t trying to kill his self per se - he was asking to be saved. He was tired and depressed. Getting harassed, abused, deep in debt, and sibling in poor health just makes it a losing game much more being betrayed by the person he trusted the most. Siyeon wasn’t in a good headspace - like every other people who go through with it they need a good support to be able to keep holding on. He doesn’t have that. He wasn’t selfish, he wasn’t in peace and dying sometimes becomes very inviting when you aren’t. It is instant block out from all the problems you have. Unfortunately, he lacked the right people to give him the best option and we know what happened next.

So you go kill yourself without a thought for you sister…but that jeha guy…yeah