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.
let's not forget a couple of things:
1) Darryl has love for his brother -- certainly such that he was ready to go right back into the city to look for Merle.
2) Glenn can NOT keep a secret. The truth about killing Merle would have come out eventually. And it would have been an intense mess once Darryl would have found out that Merle was killed.
Was Merle that much worse that Carol's husband?
And Glenn's personal intercation with Merle might not have been that bad...for all we know, it was Glenn who chose Merle to be part of the team that went in to get supplies.
I think what happened was consistent with Glenn's character..i'll know he'll be hurt, but i think he'll survive Woodbury. If not -- i will be VERY sad!