Ok. But not many people would pick the heroic move. I, personally, would probably pick my own comfort over certain death. If he hadn't joined the Death Eater and tried to defy Voldemort, he would've been killed.

This thread is about who is the real Slytherin hero though. And he wouldn't have been killed if he hadn't joined the death eaters. Defying them could get him killed, but he could have just sat by and left the death eaters alone.

Riiiiiiight. One of the few pureblooded families is just going to 'sit out' being in the Death Eaters.

Everyone else in his immediate family did.

So, Regulus, who grew up in a pureblood supremacist family that worshipped Voldemort once he became big, was simply supposed to ignore them? It's how he was raised.

Sirius explains that his parents agreed with what Voldemort was doing, but they weren't actually followers of his. They were proud of Regulus for joining the Death Eaters, but it didn't sound as if that was necessarily expected of him.

For the actual question Regulas didn’t really do anything but fail to kill the horcrux (locket) and make Harry’s job harder by having to track it down. Snape at least did something productive in the fight against Voldemort. (The question itself I am interpreting as “what hero that happened to be in Slytherin did more” instead of “who is the hero of Slytherin house and it's ideals”)

Well, if we are talking about the "hero" of Slytherin's ideals, then obviously its Voldemort.

Er...how?

Well by joining Voldemort he was able to learn about the Horucruxes. If he didn’t join he wouldn’t have found out. Harry and them wouldn’t have been able to destroy the locket. Voldemort would have won.