thank you for your lovely tags @lothiriel84 !
that's the question, right? how does their story end? – which is just another way of asking: how does david's story end? (setting aside the possibility that everyone snuffs it at the end of s2, in which case david's individual arc still needs a conclusion)
as i see it, any answer to this question is made up of two components. (1) there is the what actually happens. (2) there is the why – i.e. what meaning does (1) carry? what does it represent for david and his character development?
so let's address (1) first and start with the easiest answer: david finally rejects & defeats colin in some way, e.g. by kicking him into a black hole. imo this is by far the most likely scenario because it's dramatic, it draws a line under the story & the characters, saves the day, and it has the potential to satisfyingly resolve david's arc in a number of different ways, depending on how it's handled. it also draws a BEAUTIFUL parallel with the defeat of the comfort buddy in s1e1. as i see it, as long as david has not permanently rejected colin, then their story is not really over. (if this doesn't happen at the end of s2, then it happens during s2 and the remainder of their stories are about figuring out how to live without each other. but for now let's assume it happens at the end.)
so if that's the (1), what is the (2)? what does this represent for david's Journey?
let's go route one by fixing what i reckon is his most fundamental character flaw: he has developed self-respect, and is no longer desperate for external validation. the gag ending i would pitch here is that by kicking colin into a black hole, david becomes a hero and is immediately SHOWERED in cartoonish levels of external validation.
in a similar vein: he has learned to recognise & value what genuine friendship looks like. i think this version is less likely, as marscorp is not exactly hot on positive interpersonal relationships, least of all friendships. as of HC, the martians certainly don't treat him well enough to justify this ending, and frankly neither does hob; the robots arguably do, but they'd need to put in a lot more work to get to a place where you could hang a satisfying ending on their friendship. it does say in the s2 preview that david has made "a new friend," so we will have to wait and see what that means.
(the exact status of colin & david's ""friendship"" continues to be extremely complicated and challenging. i'm not even sure how to begin unpicking it. suffice to say that i think the show & characters are likely to continue to frame it as essentially a friendship, regardless of what goes on romantically/sexually.)
now, alternatively, it could be that the (2) is about morality. as time goes on, we see more and more glimpses of david's evil side, so maybe s2 is really about testing his character in this respect, and his arc concludes with him choosing to be A Good Person. (maybe this doesn't square so easily with kicking colin into a black hole, so maybe this (2) corresponds to a different (1) – rejecting/foiling colin in some conclusive but less definitively fatal way.) i think this is a less likely route, as Being Evil is really not as central to david's character as his desire for validation & friendship, but it's still possible.
these are my picks for neat, happy(ish), route-one endings. so needless to say, there is a high probability of dh doing something wildly different with it, or setting up one of these endings only to undermine/reverse it somehow. or of course, two or more of these angles could very easily be combined. if you have any thoughts about these or other possible directions, i would LOVE to hear them :)c but the short answer is yes, i think david will have to kill colin eventually
