Some people have learned Visine is poisonous.
They believe the cops are not familiar with the signs of Visine poisoning, and they think authorities won't know how to test for it. Similar situations have arisen when the public learned of new poisons (usually in crime fiction, or in news reports of unfortunate accidents). I remember after a picnicking family was killed in Florida by roasting hot dogs on oleander stems, a rash of attempted poisonings using oleander leaves (they look like Bay) was reported.