The Internet says that platypus venom can kill small animals, but I haven't come up with a definitive source for this.
There is an informative Wikipedia page about the venom here. Basically the venom contains a number of toxic peptides including a family of C-type natriuretic peptides, some defensin-like peptides and nerve growth factor. Defensins are very widely-distributed in nature but typically are used by cells as antimicrobials. Natriuretic peptides have a range of effects upon fluid balance and blood pressure control - these are the best-characterized components of platypus venom.
These peptides are all genetically-encoded, so they are made as segments of precursor polypeptides. Intriguingly some of the the C-type natriuretic peptides contain a D-Leu residue. (All of the amino acids used in protein synthesis are the L stereoisomeric forms.) The presence of a D-amino acid in a peptide derived from a genetically-encoded molecule suggest that there must be a specific isomerase present, and evidence for this isomerase in the venom has indeed been found as reported in Torres et al. (2006) Mammalian L-to-D-amino-acid-residue isomerase from platypus venom. FEBS Lett.580:1587–1591. The introduction to this article states:
Platypus venom contains many non-protein and protein components...whose roles are yet to be established.