What is this image from? Is it Thing Thing Arena? There were so many of these kinds of games on Addictinggames.com when I was a teenager.
This is from a Flash animation, not a Flash game. Specifically, it's from SACRIFICE.fla, an instalment in the Madness Combat series. The screenshot is cropped from the opening scene, in which the protagonist Deimos is freefalling through
the afterlife, following his death in an earlier instalment.
Madness Interactive is also part of the Madness Combat series, so I can understand why Bassie thought that was the answer, but Madness Interactive came out in 2003 and predates Deimos' debut by several years.
(Content warning: SACRIFICE.fla contains extreme graphic violence and disturbing imagery.)