Killing your alternate self takes the life force they could have potentially spent during their lifetime and divides it among the survivors. However this life force fades with age. This would explain all those stories about 75 year olds who climb Mount Everest and such. They've gotten the life force of all their copies that they outlived, but they're so old and the amount of remaining life force so little, that it only makes them a bit more active instead of a super-powered being. Yulaw on the other hand is deliberately killing alternate versions of himself when they're decades from expiring naturally, giving him massive amounts of power whilst still young.
In addition to the above posters, most people not only die old but few die in rapid succession. The One spent several years traveling and killing people at the peak of their lives so it makes sense he becomes strong because he accumulated the strength of at least a 123 people in a short time. Even if they were frail and old, it would still add up to an impressive amount especially considering the next point below....
A few versions of people who die are Badasses in the peak of their lives. Yulaw was killing not only his other version in their youth and in rapid successions, but he also killed some very Badass incarnations in his 2 year spree as seen in the prisoner Lawless. There are at least two other people implied to live dangerous lives such as the crime lord and a few young people who look to be athletic shape. So you're getting some people who are extraordinarily tough and badass and even the "crazy ones" as Yulaw calls them are athletic or at least not unhealthy enough to be frail and skinny or obese. When you combine the fact the life force disperses, it not just gives accumulated upgrades, it literally multiplies the amount of strength Yulaw get each time he kills a person. Which means an already Badass person would have at least 2-3X the gain in life force as Yulan continues killing more. Hence why he's almost god level.
The whole "alternate universes with a different version of you in every one" aspect of the movie naturally brings up a major Fridge Horror consequence that's present in every take on the idea: Everyone's lives are predetermined. After all, how else will you get together with the same guy/girl to have the exact same kid in every universe?
Granted, this ignores the possibility that it is simply most probable that one would make such and such decision at a time. The fact that there are finite versions of Law doesn't necessarily mean there are only 125 universes, but simply that there are 125 iterations of Law that are similar enough to be connected.
Yulaw is last seen defeating scores of inmates in a prison dimension. However he's actually still under the effects of being teleported. In a few minutes or less he'll be back to full strength.
Even worse: if he's already killed 123 of his own duplicates, that means he's been there before, to kill the duplicate of himself that was native to the Prison world. In which case, he knows how to escape it.
To be fair, the Hades!Law might have been the first one he killed, meaning he would have had his quantum tunneling device at the time, The EU website explains that he was found out by the MVA, when he lifted a box full of weights like it was nothing, by accident, he would have had to have at least a few Alt kills under his belt to do that.
It may also be worth considering that there may not have been a version of him on the Hades universe anyway; as previously observed, just because there are 125 versions of Law that he could kill and absorb doesn't mean that there are 125 universes, but just that there are 125 universes where he exists in some form.
And considering it's a prison colony, it's possible that the Hades Universe doesn't even have a native human population: that everyone there is a prisoner or warden brought in from some other world. Or at least, given how inhospitable Hades' sky appears in the glimpse we get, that the terrible conditions keep the number of native-born inhabitants too tiny for most people in other universes to have counterparts there.
Gabe might seem like he's gotten a happy ending by being sent to a different universe. However, he's in a place completely unfamiliar to him and has no job, no family, no friends, no home. The closest thing to happiness he has is that universe's version of T.K and as the movie makes clear, having the same face as another person does not mean you have the same personality or history. Gabe might not be in prison but he's arguably in a far worse hell than Yulaw.
Tying slightly into this is the fact that the universe he is sent to at one point might have had it's own version of Gabe, and when he gets fingerprinted or the like, we don't know how exact the Alts are to each other, that version's Law might have been killed a few years ago, with family and friends all who were probably at his funeral, he might have a lot of explaining/lying to do, but on the upside if he gets his story straight (kidnapped, faked death or something) he might get a whole prebuilt life for himself.
However things might get sticky if he finds out he has a wife/husband in this universe who isn't T.K.
The evil version of T.K who helped Yulaw escape is still out there and there is the possibility she can aid Yulaw again.
How can the MVA not know the effects of someone outliving all their alternate universe selves? Surely out of all the billions upon billions of people who've ever lived, at least one of them had to have done it without having to go on a multiversal killing spree. And for that matter, if a person's remaining life force is redistributed among the survivors, why don't we have a lot more super-powered geriatrics?
What happens if every version of an individual dies... at the same time? This has to happen eventually, by age and, by the other side, the "Fridge Brilliance" supposed that natural death does not give energies. But what if the two last versions of the same characters destruct - like in the film - are able to destruct themselves reciprocally and simultaneously? See Wild Mass Guessing, I suppose.
