@Xeli
His poison grenades are much more efficient and faster. He's not going to stop then all with a melee unless he has a weapon that can cleave then entire wide hallway
Sure gas grenades are probably more efficient (not that they straight out killed anything though), but a melee weapon instead of an SMG is probably more effective in a situation such as this. Warframe has been referenced in numerous comments but a glaive or some big zweihander wielded by someone in an exoskeleton is fucking ideal in these packed hallways.
The MC is really weird with his CP, he goes from wasteful to super conservative at the press of a button (in general not just this instance). And mini spoiler, no he didn't learn a lesson from the harpy mission.
Time = money = CP, every situation has an ideal amount of firepower needed to resolve it in a timely fashion without going completely overkill but MC rarely hits that sweetspot and wastes either time with shitty weapons or CP with more force than needed. Of course going overboard is preferable to coming out short in dangerous situations, but he even fails to do that.