Is it against the law in any way and could you get arrested/charged/etc replying to a YouTube comment saying “go kill yourself”?

I'm going to move this question out of the Libel & Slander forum to the Internet forum, which I think is most pertinent to your situation for reasons I'll explain in a moment.

But your question as phrased actually implicates criminal law when you asked about being "arrested/charged/etc." As to that, lawyers who regularly practice criminal law are better suited to give you a confident answer than I am as a civil trial lawyer. However: I will bring to your attention the recent criminal conviction of Michelle Carter for manslaughter based on text messages she'd sent to a boyfriend, urging him to kill himself, which he did. My impression is that case was an extreme outlier, rather than anything typical: the victim was particularly vulnerable, Ms. Carter was alleged to have known and exploited that, and he did indeed act in conformity with what she was urging him to do. Link: https://abcnews.go.com/US/wireStory/michelle-carter-texting-suicide-case-leaving-jail-68474770

Your question doesn't include anything remotely close to that kind of situation, so even if some Texas statute might likewise support this kind of prosecution, I would expect the odds of it actually leading to you being arrested and prosecuted are quite remote.

Urging suicide isn't exactly defamatory — meaning, a false statement that damages someone's reputation — so even on the civil (non-criminal) side, you're likely not at much risk for being sued for libel (written defamation). Some states allow a victim to sue someone who's intentionally inflicted emotional distress upon that victim, but in Texas, that's defined so narrowly that you're probably at very low risk of being sued successfully under that theory either. In Texas, there's a ton of stuff that amounts to rude or bad or shameful speech, without being so bad that the victim has a plausible lawsuit.

Your main risk probably relates to the terms & conditions fine print in whatever social media or other internet platform you used for these communications. If the platform host (e.g., Facebook or Instagram or a blog or online magazine's comments section) is a private entity, and it bans you or restricts your privileges based on its decision (whether right or wrong) that you've violated its standards of behavior, you're probably stuck with that result.

I'm *a* lawyer, but not *your* lawyer, until we've documented my hiring through a written Representation Agreement signed by us both. The impressions given here are to help you decide whether you should consult a lawyer directly; don't rely on my impressions or opinions for any further purpose.