You are probably correct, but, you're jumping to conclusions that Glen has never seen another side to Meryl. Meryl was a member of the Atlanta group before we came along, we have no idea what he was like in camp before the roof incident.
As far as Rick having a far different reaction, I don't think Rick would've shot him straight off, I believe he would've given Meryl a chance to make a threatening move before killing him (Which is what Glen did, but, Glen was unlucky/not as much of a stud as Rick)
I think it would have been a far different reaction if he wasn't Daryl's brother as well. Almost no one has relatives anymore. I don't think the group would have just killed any of the survivors relatives witout hesitation.