Who is in Slytherin?

[Liz Hildebrandt]

No: HPFGUIDX 81522

(Apologies if this topic has been covered; I made a
cursory search but I haven't yet completely figured
out how to navigate this groups.  Also, hi.)

I've been trying to comprehend Slytherin.  It seems to
just be a respository for nasty people, but what
exactly is it supposed to be?  According to the
Sorting, Slytherin people are cunning and ambitious
and probably ruthless.  Yet according to the history,
Salazar Slytherin would only take in purebloods.  That
was his thing.  The password in year 2 was "pureblood"
so that must still be active in some way.  

It doesn't seem that these two things would coincide
all the time.  Anyone can be cunning, ambitious, and
ruthless, and it does tend to happen in people who are
not privileged.  How do they deal with that?  Is
there just a lot of in-house racism?  Yet there seems
to be loyalty (to some extent) within the house and
you never hear of a non-pureblood in Slytherin.  

Was Tom Riddle Slytherin?  (It might say somewhere; I
can't recall.)  If so, how did he get in?  

We've seen only a few Slytherin characters and so far
everyone who is cunning, ambitious, and ruthless also
happens to be pureblood, nasty, and racist.  (Snape is
a possible exception.  It's hard to tell what's going
on in his head when it comes to the pureblood issue.) 
So what does make up a Slytherin?  Who is chosen?  

Liz