I know that one of reasons is that Malfoy would be killed by voldemort but if they helped dumbledore to survive they could help Malfoy as well.Voldemort couldn't kill harry because he was hiding so they could hide Malfoy as well.And severus didnt need to be in voldemort's favor.They could just all survive.
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Answered by Aslan
I'm not an expert on ths as many are, but I recall that the curse of the ring was already killing Dumbledore. I thought that Dumbledore asked Snape to kill him.