Thallium works just as well, is just as incurably fatal, slow, painful and very few survive a stiff dose. Plus thallium poisons are much harder to trace than polonium isotopes.
Pardon me for butting in, but there is a antedote for either radioactive or non radioactive Thallium poisoning. It is named Prussian Blue.
Link: http://www.fda.gov/cder/drug/infopage/prussian_blue/Q&A.htm#10
The key is "can be traced".
The killer could use polonium traces to depistare.