There are only three burials for Slack/Slacke that I can find on the Island before 1900:

SLACK, William infant Santon 3 Mar 1797
SLACKE, John Gray 39 Douglas St Georges 29 Aug 1847
SLACK, Louisa Maria 36 Douglas St Georges 26 Jul 1848

I would guess that people dying in Douglas in their 30s in the 1840s would be as a result of a Cholera or similar epidemic, a good reason to emigrate!