The actor told Howard Stern why the character of Susan, George Costanza's fiancée, had to die.

Jason Alexander and Heidi Swedberg in Seinfeld. Their faces also represent how the actors felt working together.
(via NBC)

Seinfeld fans have long discussed the death of Susan Ross, George's longtime on-again, off-again girlfriend. The characters became engaged at the start of season 7, much to George's chagrin. The arc continued for so long that the fans assumed they were going to be married. Then, in a major twist, she died in the season finale, poisoned by cut-rate wedding invitation envelopes bought by George. It was a shockingly dark turn for a show that was already one of the darkest comedies on TV.

Sources: Uproxx