::sigh::
If you want to start comparing other spells, let's look at the numbers.
The baseline save-or-lose spell (Slay Living) is 5th level. It requires a touch attack, and allows a save.
Look for an equivalent targeted spell, and you have Finger of Death at 7th level. It looks like the touch attack requirement is worth 2 levels of spell power.
Take away the saving throw from FoD, and you get Power Word, Kill-- another two levels higher. However, note that PWK has a cap on its effect; it won't work on anything over 100 hp. Apparently removing the save is worth more than 2 levels, because they had to add a limitation too.
Now for the deductive part: start at the 9th level and work backward. Take PWK and make it require a touch attack; that's worth 2 levels. You end up with a 7th-level spell with no save, that kills a touched target with 100 hp or less. That sounds like a fair spell, right? Well, Harm is effectively the same thing, yet its effect has no cap, and it's a level lower!
Maybe if it were 8th level, you might be able to argue that it was balanced against other spells. (Some would still contend that a saveless, uncapped "save-or-lose" spell should not exist in the game, but that's more of a philosophical difference than a balance issue.) As written though, Harm is clearly an order of magnitude more powerful than all other 6th-level spells.