Would house-mates ever hate each other?

[tish]

No: HPFGUIDX 60433

I take it the theory going around is that, because of Hagrid's 
statement that there wasn't a wizard that went bad that wasn't in 
Slytherin, that James, Sirius, Remus, and Peter were all in 
Slytherin.  I can see how that would be possible, but then why would 
Snape hate them?  Remus has already said they and Snape were in the 
same year (never said the same house) and they didn't like each 
other.  If they were house-mates, wouldn't Snape have seen them 
sneaking out every month?  Wouldn't he have noticed that they were 
not in their beds at night?  If Harry shares a room with Ron, 
Seamus, Neville, and Dean, then Snape surely would have shared a 
room with James and the bunch.
When Remus tells H,R,& H that Snape was very interested to know 
where he went every month, I assumed he noticed Remus was not 
present in classes or around school.  I didn't think he made it 
sound like he was missing around the Slytherin common room.
I would have to believe that if they ALL were in Slytherin, that 
after 7 years of sharing a room and spending a lot of time together, 
that the bitterness would have settled down a bit.

Does anyone else see the holes in this theory?

Tish