Ut malum praevaleat, satis est non magis quam homines probos nihil facere.
I've usually seen satis est ut in constructions where the point is to say "It is enough to do X." I really don't want to say "Doing evil is enough" or lead a listener down a grammatical garden path that suggests that. So I'm not sure about ut here. Or does ut come across ironically, amplifying the meaning, since normally it would indicate an intended result?
Here's a variation:
Quod mali vincant, non magis quam nihil faciant homines probi.
I actually think quod is wrong here, but I dug through some grammatical stuff and couldn't settle it.
I'm not sure if it's comprehensible to make a subjunctive clause the object of non magis quam, but there it is. I find quam the single most confusing word in Latin. The infinitive seems more natural to me, but that might only be because I have a lot more experience with acc+inf.
This one is a chiasmus, trying to put emphasis on the contrast between probi and mali by delaying it until the end. I'm thinking that contrasting improbi and probi would sound lame.
I'm thinking here that non magis quam is clear enough to drop satis est, and omitting the main verb even adds some bite.
I went through a lot of possibilities for a noun for "evil", and none seemed to suggest a force in the world—bad, power-seeking, destructive people in the world. After all that, I'm thinking that the adjective malum might actually fit best. Without a noun, I'm think that the adjective means "evil in the world" or, in the plural, "evil men". Hopefully it's not misread or misheard as mālum or the other senses of mali, which would make the sentence ridiculous.
Triumpho is of course another option for the first verb, but I'm thinking that in Latin it suggests the triumphal procession rather than the victory. I'm thinking that praevaleo suggests getting the edge, the upper hand, taking over—which would be perfect.