Did Dooku ever personally kill a Jedi

  First to mention that we as readers know about Dooku's motivation mostly from Legends novels like Darth Plagueis and Labyrinth of Evil. There is precious little information in Canon.

In SW universe very few people knew Dooku well enough to know why he turned to Dark Side, and their number is even smaller if you count only those that survived Clone War.

Among them Palpatine is first. Palpatine probably knew most of Dooku's secrets, including his weakness that seduced him to darkness. But Palpatine also wanted to erase all traces of Sith Grand Plan once it was fulfilled, and to destroy all clues that connected him with Dooku. Therefore, it was highly unlikely Emperor would talk with anybody about dead Count, except ...

Vader, who killed the man and took his mantle of Sith apprentice. Was Vader curious about Dooku, and did Palpatine satisfied his curiosity we currently do not know. It is worth of mentioning that Vader did travel to Dooku's home world of Serenno in order to punish nobles there. This is also part of Legends, but theoretically Vader did have opportunity to learn something from this expedition.

Finnally, there is another group that could have known something about Count and these are Kaminoans. In Clone Wars TV series Kaminoan cloners communicate with Tyranus in order to keep secret of Protocol 66 and inhibitor chips from Jedi. Did they knew that Tyranus, man who ordered Clone Army, is actually Dooku ? Most likely, because Dooku was after all public face of the CIS. In Clone Wars S06E02, Kaminoans discuss with Tyranus true purpose of inhibitor chips, implying that they did have some info about motivation of their client. What happened to them after the war is unclear in Canon, in Legends they attempt a rebellion against Empire but were quickly put down by Vader.

Could anyone of people mentioned above talk to Jedi ? Palpatine almost certainly not, but Vader after his redemption and reappearance as Anakin's ghost could be potential source of information on many things. As for Kaminoans, it is anyone's guess .