That's not very fair. He had a gun trained on Merle the entire time for all those same reasons.
Glenn didn't instantly and cold-bloodedly blow Merle away — therefore he's a born sucker and he was asking for it?
You don't do shades of gray, do you?
Shades of gray are not universal--particularly when the subject (Merle) has never proved he is anything other than violent and vindictive. Glenn--or anyone else in the original Atlanta group--does not know of another side to the man, so it would not be logical to expect the next encounter to be civil. Who would forget Merle cut his own hand off?
expect a happy reunion, after the way the original situation ended? Again, Merle himself admitted there was bad blood (only promising to let it go if taken to Daryl), so Glenn was being hopelessly naive, and is now paying the price.
I think few would disagree that Rick's reaction to a new encounter with Merle would be far different, much like his immediate reaction to the inmate's attempt to kill him.
In a world where zombies are far from the only enemy, when established bad and/or violent conflicts occur, only a fool would think an old enemy was somehow reformed.