The crash of the Air-India Express Boeing 737 at Kozhikode on 07/08/2020 resulted in the loss of at least 21 lives.
It appears to have been the worst aviation accident in Kerala (counting both civil and military aircraft). The previous worst accident was near Kochi airport in 1998, which resulted in the loss of 9 lives including 3 on the ground:
One military crash had a slightly higher death toll of 10 :
There have been other accidents which occurred outside Kerala but had a large number of casualties from that state. Offhand one can think of Mangaluru 2010 and Mumbai 1978 which must have had at least half of the passengers from Kerala: