pernicious

pernicious \pur-NISH-us\, adjective:

Highly injurious; deadly; destructive; exceedingly harmful.

Half-truths can be more pernicious than outright falsehoods.
-- Wendy Lesser, "Who's Afraid of Arnold Bennett?", New York Times, September 28, 1997

But he said they were not thinkers but snobs, and their influence was pernicious.
-- Saul Bellow, Ravelstein

Racism should be condemned because its effects are pernicious.
-- Luigi Luca Cavalli-Sforza, Genes, Peoples, and Languages

Pernicious comes from Latin perniciosus, "destructive, ruinous," from pernicies, "destruction, disaster, ruin," from per-, "through, thoroughly" + nex, nec-, "violent death."