Calvin was friendly until it thought humans were trying to kill it

It just needed to gain enough strength to be offensive but that's all. As soon as it had the advantage, it became completely berserk. Considering how smart the creature was, it is clear that it was not trying to make any contact with us.

Those little shocks that it was given was not the surge of electricity you'd get from putting your finger in a wall socket, but perhaps the same as shocking yourself due to static. Not enough of a reason to kill the crew.

See how Calvin was reaching for Hugh's finger while he was still in the petrie dish? I don't think he was curious. I think he was displaying the aggression of a predator. They were all on borrowed time. They just messed it up sooner than they had to.

Nope. He was trying to survive as it was stated by the doctor.

He was playing dead so that they would do something like give it a weapon to escape with. He probably assumed they would try to dissect it and then he'd have a scalpel. But instead they gave him an electric wand which worked just as well.

Hell hath no fury like a Martian scorned.

I think one of the best things about the film is that we can never really be sure what Calvin was thinking or planning or if its plans changed at any point or what.

It was a totally alien life form that didn't behave in human ways at all, outside of some interaction that could be interpreted as a ruse, genuine friendship, curiosity, or anything.

Maybe that's normal behaviour for its species. Maybe all children hunt parents. Maybe it thought of them as pets. Maybe it was a farmer. Maybe it was a friendly little guy who turned angry when "betrayed" by the shock. Maybe it was playing dead to lure in prey and it was just a near-mindless killing machine with buckets of intelligence, but no real consciousness or purpose or meaning to its life or self-reflection at all, just hunt-kill-repeat.

Who knows? All we know is that, at some point, it attacked and became utterly relentless.

The movie was, mostly, a knockoff of Alien, but the unsettling "otherness" of the life-form was pretty spooky.