Telling about the Horcrux would ruin Slughorn?

What was the big deal? He was already an old man, doing nothing in a crappy house. What reputation did he have left to ruin?

He didn't have some high up position in the ministry or whatever to lose. If anything, it could help get him some fame for helping stop Voldemort.

He could have lied about it if he was so afraid. Say he caught Tom Riddle reading up on horcruxes. Or even send an anonymous letter to Dumbledore with that info.

The guy was ok with letting wizard hitler live just because of his reputation. What a loser.

Don't get me wrong, i'm not saying this is a plot hole or anything, i'm judging the character, not the writing.