In nearly 50 years as an artist and children's TV show host for the BBC, Tony Hart taught generations of Brits to love art. His slow-paced television programs put a gentle, kind man on Britain's main stage, where he carefully moved his pencils and paintbrushes across canvases, guiding his viewers to do the same until his retirement in 2001. Here, Murtha investigates how an errant tweet and a high-functioning algorithm killed Hart, a man who had died more than six years ago