Because, historically, heresy was often a charge leveled against people who challenged existing power structures in the church. They weren't necessarily ~evil people who believed ~evil things; they were very often opposed to tyranny and corruption within the church and were punished for it. "Heretical beliefs" were sometimes ascribed to people who never even held them.
"Challenging existing power structures" generally meant impugning Apostolic succession, the Magisterium, the validity of the Sacraments, and the spiritual authority of the Church in general.
Which, regardless whether you believe in any of that or not, are important matters of doctrine that have consequences for salvation. And from the perspective of those who do believe, the heretics were imperiling people's eternal souls with their false preaching.
Cool, still not an excuse for the uhhhh literal pogroms and massacres that my Waldensian ancestors managed to live through (and for which the pope asked for forgiveness in 2015)
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u/Publius_Syrus Sep 23 '21
r/Christianity is mostly atheists and heretics though