In the remote past, 15 years ago, I have a Sixline pair and Mandarin pair in a 420. All four fishes (Sixline and Mandarin) coexisted for at least 1 year, and were spawning regularly in my tank. One day the Sixline pair attacked the female Mandarin. The attack last about 15 second and happened right in front of my eyes. Over so quickly that I cannot do anything about it. The wrasse took one pass each, one go for the R eye and the other one go for the L eye of the female Mandarin. After that they completely ignored the blinded Mandarin, and her mate. She starved and become emaciated. There was no hope of recovery, I euthanized her after 2 weeks. I was lucky enough to caught the two Sixline and traded them in for another female Mandarin.
It is all instinct, but it seem to be instinct from Satan-Spawned fish for me. I vowed never put another Sixline wrasse into my tank again, even for free.