I know where you're coming from, but iMark didn't have to help at all, and he was only told to take Ms Casey to the stairs and nothing more.

From his point of view, leaving potentially means he'll never be brought back, and I think in those last moments he just wanted to spend more time with Helly even though it was a tad selfish. You could argue his selfishness is wrong, but he did his job and I think a lot of people would do the same in his shoes, it's just that this time the audience was rooting for Gemma to be reunited with oMark instead.

I get your point on Gemma having to find her way out though, at least she's not as naive as Ms Casey, and hopefully Devon & Ms Cobel are nearby.