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.