I was asked to do this door lorythefangirl after I responded to her earlier one.

Pros
1. Most of them did believe they were choosing the winning side at the time
2. They would likely be killed if they refused, of otherwise punished.
3. If they come from old pureblood families which share the Death Eaters' beliefs, joining up would help maintain family harmony
4. If they're naturally malicious it gives them an opportunity to kill and torture

Cons
1. Most people wouldn't want to be forced to murder. They might be pushed beyond their own limits, e.g. Draco Malfoy.
2. They'd get imprisoned in Azkaban if caught, of could be killed in self-defence
3. Leaving isn't an option once you're in
4. After they lost, those who'd survived would lose favour even if they avoided punishment
5. Voldemort doesn't seem to be a nice boss
6. It's a story really, and in most stories, the bad guys do end up losing and being punished.