Green’s Dictionary of Slang

(N.Z. prison/US campus) suicide.

1966		Current Sl. I:3 2/1: Commit sideways, v. To commit suicide.
1982		H. Beaton Outside In Act II: If this keeps up for much longer, Ma, I’ll end up committing sideways.
1982		G. Newbold Big Huey 253: sideways (n) Suicide.
2003		McGill Reed Dict. of N.Z. Sl. 189: sideways Suicide. A euphemism suggesting you are not leaving this world front-on.