As Sophie Turner pointed out in her tweet, Littlefinger orchestrated many of the plot twists that led to great suffering for the Starks.
Young Rickon would probably be alive if not for the sequence of events Littlefinger kicked off.
Even if some fans will miss Littlefinger in Season 8, it’s probably safe to say that Sansa won’t regret her decision as Lady of Winterfell to have him killed.