As you say, “Sure. We categorize heresy as a mortal sin” then logically heretics, whether excommunicated by formal action or automatically, suffer the punishment set forth in the Catechism?
If they were Judged by God to be heretics. We have evidence here that they are - and expel them from the Christian community, but that in no way holds God to man's judgement. The difference is in what we think about someone, versus what God knows and Judges.
This last sentence explains why a Christian would not take anyones life because they are deemed heretics whatever they felt God’s judgement to be upon them.
Everything Christ taught tells me we don’t have the right, the authorization or the knowledge to do so and no human can give such to another.