I'm not going to comment on the dangerous aspect, but the unethical aspect is tricky." DDOS the Russian MoD, probably fine on the surface (but maybe not, I'll get to that in a minute). DDOS the Russian's Children's Clinical Hospital, also a government site, definitely unethical.

This then raises another question, does the latter in fact depend on services provided by the former, or a similar government site that has been DDOS'd? This is possible, perhaps they are even cohosted in a data centre somewhere and taking down one will take down another.

Assuming you go ahead then at this point you are basically engaged in attacking the infrastructure (possibly civilian) of a foreign nation at a time of war, presumably from a NATO country. Now you're doing this as a private individual, and your government would probably dissavow any knowledge of your actions, but there's always the chance that the Russian's wouldn't believe you or decide to interpret your DDOS as an act of aggression performed on behest of your host nation and respond in kind, or even escalate up to and including the use of strategic thermonuclear weapons. While this may be an unlikely scenario the possible consequences are catastrophic, so is it ethical for you to risk the survival of global civilisation based on your need to be seen to do something? Probably not.

Vigilante actions are not intended, beyond justification, to be help others; they are intended to satisfy the emotional needs of the vigilante. If you are serious about helping Ukraine, volunteer for an organized, competent, serious group. I think the Ukrainian government was asking for volunteers a few weeks ago.

Acting on your own is not only likely to be ineffective, you may harm the wrong people, you may interfere with serious operations, etc. Imagine a vigilante with a gun on a battlefield - they could compromise opsec, create obstacles, become a distraction, etc. for an existing operation. Nobody wants some untrained loner running around with a weapon.

Yes. DDoS is always wrong. We should not start a culture of DDoSing people we don't like (even if the dislike is completely justified).

A nation state may play tit-for-tat with another nation state but they will assess the consequences and act accordingly.

No and No.

Your ISP might not be happy with the extra traffic though.

I think you should very closely examine whether or not it makes any sense. Put aside for the moment whether or not doing stuff like that could be dangerous to you legally. What is the goal? To temporarily disrupt computer systems based solely on whether they're supposed to be affiliated with the Russian government? What is the point?

I'm glad you're at least thinking whether DDoS could be wrong. I've never encountered a situation where DDoSers were the good guys.