Hm...well, we could go with the theory that the Shivans were created by a race of capricious gods to fight against the Terrans and Vasudans for their own enjoyment.
You bastard.
Also, I continue to be slightly caught off guard by how intelligent you all seem to be.
However...well, yes. It's entirely possible that the Shivans are simply some species that hail from a vastly different environment, and as such just have a very different way of solving problems, and very different goals. (I'm very glad to see people understanding that self-preservation is part of human programming, rather than some intrinsic quality. It bothers me when SF gives AIs that become self-aware self-preservation protocols regardless of whether they were actually programmed to have them. It's debatable just what becoming self-aware would do, but it wouldn't make them stop following their programming. We follow ours, after all.)
(This is something I think about a lot. Simply the sort of anthropocentric provincialism exhibited by most people. Assuming that everything in the universe thinks like they do (and, furthermore, that their morals and values happen to be the correct ones, and anyone who disagrees is being intolerant, but that's a whole different discussion). And it shows up everywhere, and it drives me crazy.)
Anyway, so there's that.
HOWEVER.
Narratively, I think the Shivans need a reveal. There needs to be some central secret that has a profound impact on the player. Mass Effect failed at this, because the explanation for the Reapers was ultimately pretty much just "they're some things made by some people to do some stuff". There was nothing perspective-altering about that; the explanation was very mundane.
Conversely, just to plug myself a bit here, my planned "rewrite" of the second two games involves a very different explanation that IMO works quite a bit better. Essentially, they're the parents of organic life. We learned in ME2 that the Reapers create more of themselves out of organics. The reveal here is essentially that that's the only reason organics exist in the first place -- the turian race, for example, is basically an infant Reaper. Every fifty thousand years, the Reapers come and bring their children to maturity by assimilating them into Reaper form, then seed the galaxy with the next generation. It's the idea of reversal -- we didn't create the robots, they created us. You get a new piece of information, and everything looks different -- we're children, and we think that the big evil parents are coming to destroy us, but they really want what's best.
Anyway I've gotten off topic. Point is, the heuristics stuff is fascinating, but it's not enough on its own. There needs to be some sort of perspective-altering impact.