I'm in favor of improving longevity, but sometimes there is something to be said for other people dying. Imagine a world where Stalin was still alive and would remain so approximately forever.
I don't think this is a reason to avoid research on aging, but immortal dictators could certainly be a downside.
Had a chuckle at the mention of Stalin. Made me think. I would also think, the evils would be the one who would badly want to live forever, if an option was presented.