What's the deal with the card Decimate? It destroys every permanent type other than planeswalkers, meaning it's extremely close to "destroy target permanent", but it's red/green. Neither red nor green can destroy creatures normally, right?

While red doesn’t “destroy” them, it does have direct damage as a means to get rid of them. It’s a a bend, but wasn’t a giant one at the time. We are much less likely to make a red spell with “destroy target creature” today.

Is Decimate a bend or break? (Since neither red nor green gets "destroy target creature")

It’s a bend in red.

Could I get a bit of trivia about Decimator Web?

Okay. This is actually a cool story. Erik Lauer never liked the card Decimate from Odyssey because it didn’t match the meaning of the word (to eliminate one tenth of something) so on the first set he led the development for he made a card with Decimate in its name that actually did what the word meant and he found three different ways to do it.

So is Decimate bad precedent? Is its violation of the colour pie (neither red nor green can--or, at least, is supposed to--outright destroy any creature) mitigated by the fact that it can't even be cast without also having the other targets, some of which are rather less likely to be around, particularly under your opponent's control?

Yes, it is a bad precedent.

As far as milling keywords go, how about "Decimate" or "Raze?"

I think words that simple mean destroy will be thought of a things that kill the appropriate card type. Decimate sounds like “destroy creature” and raze sounds like “destroy land”.