It's always a dilemma when evil people must be removed from power. It's immoral to leave them in control of other people's lives, it's equally true that they will never be dislodged by conventionally 'moral' means. And often their replacements will only nominally less evil.
And what's worse is that in the middle of a situation it's very hard to tell how it will come out and whether you are going to end up doing more good or harm. You may not not know for 20-30 years. For me, I think Vietnam vs Iraq are the historical counterpoints. We thought we knew what 'had to be done' to protect the western world in Vietnam but we were absolutely wrong and if we had never sent a single boot I would challenge anyone to provide a believable story of why the world would be a worse place. OTOH, the near term verdict on Iraq was that it was a unmitigated disaster, but guess what - with time in distance it's beginning to emerge that things are beginning to hold together bit by bit in Iraq maybe the door was opened for a better future there even as badly as the neo-cons bungled the management of it.