Being a Grot is not a nice life. You are a scrawny, weak little being among big, aggressive Orks who love to kill stuff and who view anything smaller than themselves as inferiors. You are constantly yelled at and kicked, forced to do their drudge work, and your inevitable death is only good for a quick laugh. Throughout your life you cower in fear, hoping to avoid the wrath of your huge superiors, doing what they say to hopefully avoid being kicked, eaten, or shot, all the while fighting with your fellow Grots over scraps of half-eaten Squigs. This often breeds spite and bitterness among many Grots over the course of their often short lives.
But a precious few escape this fate. One day a Mekboy will scoop up some lucky Grot, a Painboy will hammer some wires into the Grot's brain, and he will be sealed into a 3-meter-tall, armed and armored water-heater on legs called a Killa Kan. Suddenly he's bigger, harder, stronger, and shootier than all those Boyz who abused him, and it's payback time. The only Boyz he can't get after are those interred in Deff Dreds. Contrary to what you might think, the Mekboyz and Painboyz responsible for the Kan's creation view this initial revenge-spree of death and mayhem proudly, akin to a sort of validation of their efforts. Even after that, the Kan pilot still gets to take out his latent frustrations on anyone else the warband happens to be fighting, so everything's great for him. Of course Kan pilots are still every bit as cowardly as they were before, but that doesn't always matter when you think you're invincible.
Killa Kans are so favoured by Gretchin (I mean, who wouldn't love to control a hugeass war walker armed with all sorts of weaponry?) that whenever the resident Big Mek constructs a new one they often hold lotteries to find out who's going to get bolted in to pilot the thing. Only witnessing the actual wiring-in process can get most Gretchin to rethink their career decisions - but as mentioned above, by then it's usually too late. On the other hand, once the process is finished the Killa Kan is free to mutilate and kill his former tormentors, so that has to be worth something after all.
Questions about how the implanted Grots feed themselves or defecate are probably best not contemplated.
Killa Kans are a mixed bag on the tabletop. While they benefit from a Grot's BS of 4+ and the Dakka Dakka Dakka rule, you really want to get these guys in melee so they can chop up sum gitz with their high-strength, high-AP Saws, Klaws, and Drillas. While they only hit on a WS of 5+, each Kan gets 3 melee attacks to start with, a +1 bonus attack in a mob of 3 or more, and some of the Kan melee weapons grant an additional +1 attack as well. That's potentially 3-5 melee attacks per Kan, which isn't too shabby when each unsaved hit means an Intercessor just bit the dust. On paper T5, W5, and a 3+ save is not terrible, but concentrated small arms fire will eventually pick them apart, and many of the heavier anti-tank weapons can potentially one-shot them as well. Unfortunately, with a move of only 6" Kans will often struggle to get into melee before getting shot up unless assisted by kunnin' tricks such as hiding within the radius of a Kustom Force Field. Also, Kans are still Grots, meaning that they receive no benefit from Klan Kultures and many of the best Stratagems and Ork traits cannot affect them, and if a Kan mob takes even a single casualty it can potentially lead to the hilarious result of otherwise undamaged Kans waddling off the battlefield in terror. All that being said, if you somehow manage to herd a full-strength mob of these guys into the enemy line the results will be messy, glorious, and probably unstoppable. This is all the reason that an Ork player needs to take them.
...'course, if yew was cunnin' as ya were brutal, they'ze yer best DISTRACTION CARNIFEX, too.