Purges were necessary, that doesn't mean the executions were. Purges often meant simply expelling someone from the party or a prison sentence, not necessarily execution. What's clear when studying the soviet union, though, is the sheer siege and subterfuge targetting them from right when they first began. We can understand why they did what they did, while also understanding that if they had better resources and political stability then the better option would have been imprisonment and potentially rehabilitation.