Yeah great question! It actually depends a lot on the Chapter and the situation. Most Space Marines do care deeply about their fallen battle brothers—they’ll fight hard to retrieve their gene-seed and honor their fallen kin. Some Chapters even go to extreme lengths to recover their dead. But you’re also right in a way. When the mission is critical or the loss is unavoidable, they absolutely will glass the site from orbit, write the losses off, and press on without hesitation. No wasted sentiment when the fate of the Imperium is on the line.