I don't believe he is alive because I want him to be shipped... I remember reading all the Armored Titan and Colossal Titan theories from way back before the 'Warrior' chapter was released, and even back then, despite all the evidence, you still saw comments like 'You're trying too hard' or 'Seriously, your evidence is the Armored Titan looks like x?'

We all know how that turned out. There is alot of good evidence that shows Marco as much more mysterious than he lets on. There already exists the 'twin personality' explanation provided in manga for titan shifters who act in one way (like helping other soldiers) but also want the ultimate destruction of mankind within the walls.

If you're really interested, here's the best and most interesting evidence I found:

i) Marco insists he wants to join the military police, and he is strong enough to do so, ranking in the top ten. However, he is REALLY insistent on the fact he wants to work for the King, almost to the point where it seems like he's compensating for something. Who else does this? Berholdt and Annie.

ii) At one point during student training, Marco insists he is no good at fighting. Eren calls him out on this, saying that he finds most of the titan marks before the others which Marco admits. Most of the group cites that Marco would be a 'good leader'. In fact, this seems true, Marco does know the qualities of a good leader, it is understanding the weakness of others to motivate them towards strength. Compared to the other shifters, he is the weakest, but he also knows precisely how humans operate, something which would be essential for shifters who are seeking to exploit human weakness.

iii)If nothing else, look at where Marco is just before Mikasa arrives to comfort the still shaken Armin after Eren's 'death' in Trost. He is literally standing right by Annie and Reiner as they discuss their 'next stage of plans'.