He'd remind the rest of Team Evil that just because they view him as the goody-two-shoes bitch, doesn't mean he can't stand up for himself and for those he cares about. I don't think he could get away with killing the Master - for one thing it undermines Xykon's threat - but I don't think the Master would go crying to Xykon if Reddy gave him a few broken ribs (or worse). Actually, I think the Master would have more to worry about from Xykon in case the Lich found out he'd disobeyed him.
This is assuming Reddy's more powerful than the Master; the Master's probably a reasonably high level, but so is Redcloak. Even if the Master is more powerful, he's also an old man and physically weaker than Reddy. With a little bit of planning Reddy could probably catch him at a time when he's used most his spells and - if all else fails - knock him out with a baseball bat.
If Tiasal was raped because of something Reddy did, it gets a little more complicated. If Reddy gets caught trying to get revenge at the Master it's going to get both him and Tiasal in worse trouble. In that case, he might try to get revenge a little more stealthily, althought I don't know how. But whatever he did, he wouldn't act unless he was sure he wouldn't get caught.
Did he even get a grave?