Neither simple nor Christian. They were leading true Christians astray. St. Paul was absolutely correct.
The real question is; was it justified by the teachings of Christ (and the early Apostles?) to lay into their necks quite literally with battle-axes and other implements of death and destruction, for the "crime" of opposing the Latin church?
You say "they were leading true Christians astray". there is no evidence for that, other than the Waldensian growing opposition to certain doctrines of the Latin church which they themselves [Waldensians] had scriptural foundation for.
Jon Huss, for similar reasons, was himself burned alive for alleged crimes of opposition, to this very same Latin "church".
It is only by convoluted reasoning and application (misapplication?) of "tradition" that brought those of that time to the point where they concluded murder was sanctioned.
If one is to invoke Paul at this point (and I do know the scriptural passage alluded to) then it would be like saying Paul sanctioned such "destruction" to be visited not only upon unbelievers, but also those actively preaching Christ, but disagreeing with some aspects of the Latin church, with such destruction directed to be visited upon the offenders, by members of the Latin church, themselves.
There is a religion in the world today whose texts clearly instruct them to slay those in opposition to their own religious teaching, and it is not the Christian one.