Trouble beating the huge guard in “Gone Missing” level

This guy is the one I’m struggling with. I’m playing on a Mac btw. I can get him weakened okay but then I see the kill symbol over his head - and from walkthroughs I’ve looked at, I need to use a melee attack to finish him off. The problem is when I run over to him and press F (for melee) it still only lets me aim my weapons, not use them to attack melee style. What am I doing wrong?

You must have set the key F from melee to aiming that's why Lara isn't meleeing and is aiming towards the tall fellow. Go to setting and then go to button mapping and then map the melee to F key. You can also map 2 keys per function in TR 2013 so you might have F for both melee and aiming so you'll have to unbind F from aiming.