Because when you beat them, you're going to take your new nazi-powers and ideals and use them against non-nazi's. It's only ever worked that way and you convincing yourself youre the exception is the first step.
Do you believe this is the first time that anyone has taken down a website through hacktivism, or that "the nazis" have never done anything of the sort?
What "not nazi-things" specifically do you suggest would be effective for fighting nazis?
You'll probably say "simply debate them on the merits" but that doesn't work.
And it's odd how in this argument the slippery slope is only ever a danger when people fight nazis, and never when people allow nazis to gain power and proliferate.
I mean I think if Hitler had confined himself to a bit of light vandalism, people might have been somewhat less upset about him.
If they were successful millions could have been spared.
The future is looking primed for war, what if millions more could be spared at the cost of one life?