Pong Krell is a four-armed Besalisk Jedi Master and General of the Republic, and the antagonist of the four-episode Umbara arc in the fourth season of Star Wars: The Clone Wars (2012). He is the rare Jedi villain whose fall the audience is never shown, only inferred from what he does.
The arc is about the clones
Krell takes command of the 501st and issues orders that are not merely harsh but arithmetically wrong — frontal assaults into prepared positions, refusals to use available cover, and eventually a manoeuvre that gets two clone battalions to fire on each other in the dark. The arc is told almost entirely from the troopers' point of view, and it is the series at its most serious about what the clones actually are.
His reasoning
He has foreseen the Republic's fall and wants a place in what replaces it, and his plan for securing one is to destroy enough Republic strength to be worth recruiting by Count Dooku. It is a careerist's atrocity, which makes it worse rather than better.
Executed by a clone
He is taken alive by the troopers he commanded and shot by Dogma, one of the most rule-bound men in the unit. The series does not treat this as a triumph. It treats it as the moment the arc has been arguing toward: an army designed to obey, having to decide.
Canon status
Canon. Krell was created for the 2012 arc and has no Legends counterpart.