Snape is loyal only to Snape.

[gwiddy2000]

No: HPFGUIDX 159686

Could it be that Snape wants to be the Dark Lord himself.  He has heard
the entire prophecy and knows that Harry (as the one marked by
Voldemort) is the only one that can kill Voldemort.  Snape give only
the information that he wants to Voldemort knowing that he will fail in
his attempt to kill the child in the prophecy and mark that child as
his equal. Voldemort is damaged, but not dead. So Snape joins
Dumbledore. Because he knows that Dumbledore will not give up until he
knows for sure that Voldemort is dead.  There is no better place that
could be.  He will be on the inside of the search of Voldemort.
Using Dumbledore as his watchdog.  Waiting for his turn to rise and
kill Harry.  After Harry has killed Voldemort.  And then
he will be the New Dark Lord.


gwiddy2000