Did your opponent just chase against the odds and suck out on you, taking a significant portion of your stack?
If so, you should be glad. That player has shown themselves to be a weak player. Don't try and get even immediately, as that is simply part and parcel of going on Tilt. Hold your fire. Wait your moment. Pick a good spot to trap them into putting all of their chips in.
Donks usually perform three purposes in tournaments...
1. They build your and other good players' stacks.
2. They knock out over-aggressive good players early to mid tournament that might've gone on to beat you.
3. They encourage Looser play, which benefits the Tighter or more Neutral players.
4. They put your opponents on Tilt.
5. Yes, they get lucky sometimes and knock you out against the odds.
However, this still means you have four things to thank Donks for and only one thing to despise them for!