Why didn't the Church just kill all the scientists like Da Vinci or Galilei, during the Renaissance? I know Galilei was house arrested, but why the Church didn't simply kill him and take his wealth? Neither of them was powerful noble or supper rich, so what protected these scientists?
Why didn't the Church just kill all the scientists
Oh, you mean the priests? Like, the Catholic priests and monks who made particular contributions to science? The same Church that outright admitted that, thanks to Galileo's observations, the old Ptolemaic model really wasn't working and thus looked for new ones?
The problem is that you're starting from a very odd position, especially since 'scientists' (the label did not yet exist at the time) and the witch-hunts are...two entirely different things. If anyone says 'Giordano Bruno', I invite you to consider that he was excommunicated from the Roman Catholic and Lutheran and Calvinist churches. Honestly, as heretics go, Bruno is way overrated.
Since this post comes from a 'Church held back science' angle, here's some previous posts chewing on The Medieval Catholic Church Versus Science:
Also, see next post.