I think the real issue is the fact that that characters with instakill level 3 supers (plus Fat Princess) have level 2 supers that are so much more effective that it makes their level 3 supers redudant and situational. Not to mention the fact that saving for the level 3 super means you would get two level 2 supers which compounds the issue even further.
Fat Princess's level 3 super takes the cake, no pun intended. No control over the super, relatively long time to reach level 3 AP bar, will almost always get you only 3 kills, and an amazing level 2 super to boot. The only situation I found it to be more useful than the chicken was in Sandover. It's possible to win with only the minion army in Sandover but it requires you to play perfectly, go 3-0 and prevent opponents from getting multiple kills.
Kratos can obtain up to 6 deaths (level 3) for the same AP's points that Fat Princess that alone can obtain 3 deaths (level 3). This is injust.
It actually doesn't work that way. All characters in the game build up AP and supers at different rates, so one character's level 3 super might be more expensive than another character's level 3.
That's how the game's balanced; every super in the game has a certain "cost" associated with it depending on how many kills players usually can get with them.
Plus this sets up for some interesting 2v2. If I'm Dante and my partner is Spike then Spike can save up for his level 3 and get that guaranteed 2 kills while I use my level 1 or 2 consistently to keep raising the score.
Then I look at kratos lvl 2 and i'll be so happy if I can pull of a 3 kill