When did HK ever kill any Jedi? Before you bring up him having knowledge of killing Jedi...ok great, but that doesn't mean he's killed any. He's a droid, they can be programmed with knowledge ya know....unless I've missed something, there isn't any source saying he killed countless Jedi. He was just used as an assassin droid by Revan.
Now he could have killed Jedi sure, but nothing has been stated as such and if he has, we can't just assume that they were Jedi of high caliber or anything. Besides he didn't face the SWTOR protagonists alone in either fight, he had help in the form of Foundry droids and auto turrets aboard Malgus' space station.
Also Battlemaster is more than just a title, the guys are some of the deadliest individuals in the galaxy with a blade, the most physically dangerous of all the Jedi, so on and so forth.
Also I'm not sure why it's a slight against Cin to him dying against Anakin, who was able to best Dooku via tapping into his rage, which Anakin was during the Temple attack, harassing his rage and being more ruthless with his moves.