unsachlich werden — to stop being factual.
persönlich werden — to get personal.
Das ist nicht persönlich gemeint. — That is not meant personally.
das Totschlagargument — literally a manslaughter argument. Duden defines it as a pseudo-argument made on purpose, hard to contest because it is empty or general, and meant to end the discussion of the substance.
die Schlammschlacht — a mud battle. Duden: a dispute, especially in politics, conducted without objectivity and with disparaging remarks.
ein Schlag unter die Gürtellinie — a blow below the belt. Duden: unfair, impermissible behavior.
Der Ton macht die Musik. — The tone makes the music. Germans use the proverb when bluntness has become rude.