And: If somebody cowardly outnumbered me four to one and tortured me in front of the whole school, just for the fact that I exist,
Do you remember these words... did you think people would take your opinion any other way with wording like that. and yes, I take almost everything dead serious. I'm kind of like that...dead serious most of the time.
would their punishment have been signing Lockheart's fanpost?
I couldn't think of a better punishment for anyone then being stuck in a room with Lockheart for so many hours... I'd have attacked him. In their first year, she gives all four of them the same punishment. Also, granted, Dumbledore and McGonnagal are lighter on Gryphindor's than other houses; however, Snape flat out ignores any wrong-doing on Malfoy's part. He doesn't just lightne the load, he altogether destroys the load. He also goes out of his way to find reasons to insult other students outside of his house; you can't tell me that he hasn't been creative in his attempts to make the trio miserable
If Snape had tried to feed Sirius to a werewolf, would it have been laughed off as a childish prank?
The difference is in motive. Sirius probably did think it was nothing more than a childhood prank and a good way to scare Snape (Sirius isn't exactly the sharpest knife in the drawer), while Snape would have done it intentionally hoping Sirius would get mauled or killed. He and Draco do have that sort of personality where they actually want someone to get hurt
Are children of Death Eaters not even worth trying?
Pretty much, we've seen how they act. you're also assuming Dumbledore hasn't been trying and that being in Slytherin doesn't mean that someone is prone to certain habits. Now granted, being in Slytherin doesn't entitle you to be a death eater, but if you read the books, it is the Slytherins who are usually the only ones doing dark stuff outright. When Cedric died, it was Slytherins who were the only ones sitting down.
Now the dumbledore point; in every book I've seen where he's trying to turn people against the dark lord. He isn't only speaking to Gryphindors or Hufflepuffs or Ravenclaws, he never excludes Slytherins. Instead, he says that the school has to be united against the Dark Lord. The only problem here is that The only contact with Slytherin's that Harry has had is with Slytherins who either side with Malfoy, or are the children of death eaters (and yes, they are NOT worth trying since it seems that their parents have done a fine job corrupting them). there are thousands of students in Hogwarts, so JKR can't possibly write about al of them in 7 books, and Harry isn't likely to notice these decent Slytherin's since their paths don't cross much, therefore, they are left out of print
In the other houses, Harry only knows a handful of people from other houses, and the most noteworthy of the Slytherins are Harry's enemies, this is why things seem the way they do. If you read the end of the fourth book, there are some Slytherin's standing to salute Cedric Diggory, and it seems like a small groups of people working with Malfoy in the inquisitorial squad, but it seems that indeed a lot of lousy wizards do pop out of Slytherin. Death eaters and dark wizards are going to take Salazar Slytherin's words to heart and impress them into their kids, so when they go to school, they want to be in Slytherin. They go home in the summers and listen to their parents be dark wizards, and when they leave school they become dark wizards
As far as people changing is concerned, a leopard never changes its spots. Snape may not be working for Voldermot anymore, but he is the same he always was