Certainly, no Leninist denies the need for periodic re-registrations of party members (i.e. purges). Lenin put this policy in place in 1921 in order to ensure that old Tsarists, SRs, and Mensheviks who had come over to the Bolsheviks for opportunistic reasons were excluded from the party.
This 1921 purge was successful in ensuring that the party could successfully undertake collectivization, the first five years plan, etc. In 1937, after two five year plans and the effective smashing of kulak power, the party needed to be purged once more, this time of opportunistic kulaks and their supporters who came over to the Bolsheviks, like so many entryists before them, when they realized who was winning.
It's not a surprise that these Kulak elements would link up with party rightists who had opposed the five year plans. Therefore, this element needed to be eliminated from the party membership.
With hindsight we might say that expulsion would have been enough, that these people did not have to be killed. However, put yourself in the Bolshevik's shoes. You have a rightist, pro-Kulak tendency in the party. You have reason to believe that this clique is linking up with Trotsky internationally who was calling for open counter-revolution in the USSR. Add to that the growing wave of European fascism preparing to unleash the blackest terror that working people had ever seen. We should not forget that Mussolini was a self-described "apostate to socialism" who was once a member of a rightist faction of Italian Socialist Party.
Whether they were incipient fascists or not, the Rightists in the Bolshevik CP were a threat of some sort. That Stalin and the majority acted with caution, ended these people, and saved Soviet socialism is proof enough that their actions were both understandable and justifiable.