Ignotus stands out from his two other brothers as not only the youngest but also the wisest. And he does not seek to overpower or humiliate Death, simply to avoid him, or, in a more realistic setting, just to be left alone and protect the ones he loves

Similarly the Imerius curse stands apart from the other two unforgivable curses. While all three can be used for evil and ae very dangerous in the wrong hands, the experience of being hit by the other two is very different to being hit by the Imperius curse. Avada Kedavra obviously kills you and the crusiatus curse causes imence pain and suuffering but the Imperius curse makes you feel a sensation similar to floating as every thought and worry is wiped away, leaving nothing but vauge, untaceable happiness.

So rather that death or pain, it's bliss. And, of course, complete controle over the other person. But if Ignotus is indeed the crator of the Imperius curse, the consider his intended use, it's just to hide and protect. He never had any intention to kill or cause harm or, in this case, inslave others. his intention was to just turn people away in a kind, almost peacful way. Obviously it was put to terrible use in the end but can you really hold that against him when it wasn't the original use for the spell? 

Obviously, when in such a state of unquestionable happiness, it is very easy to be controled. As once hit by the spell, the one who cast it simply tell you what to do. You hear their voice as though you mind is an empty champer and the are in the deepest part of it dictating your every move. 

It is not, however, impossible to ignore these comands. While it is much easier to just obey the one who cast the spell, it is possible to simply say no. While there telling you what to do you can just decide you don't want to follow their instructions, however, it is a lot easier said than done.