Ending of Whiplash...revenge or not?

Fletcher planned the entire thing, so he would seemingly be fine with ruining his career. However, it wouldn't really ruin his career because as was shown several times in the film: it's the musician's job to know the songs and play them correctly.

It's really more of a mix, and one in which Fletcher really has nothing to lose as he's already lost his prominent job. In an earlier scene we see him telling Andrew he pushes his students to exact greatness from them, even providing anecdotes of other great musicians who were pushed hard and turned into greats.

So really, Fletcher can't already lose more than he already has, and can write off the incident as his drummer not practicing hard enough, making it damn near impossible for Andrew to realize his dream while getting revenge on him as well, while simply carrying on as usual.

Andrew realizes that Fletcher is in a no lose situation, so he decides to head back out and hit him back by playing off-song, stealing the band from him at the same time. In the end, however, he proves he's one of the greatest drummers of his generation, and provides Fletcher with what he's been seeking all along.

The end of the film really is rather morbid in some sense, as Fletcher's methodology is fairly unorthodox and downright cruel. However, the main point is that greatness sometimes comes from being pushed to your limits, and coming back after being knocked down.

In short, Fletcher really had no way to really "lose" or "destroy" himself with how he set Andrew up, only Andrew did.