Is ending words with 'ised' versus 'ized' just a UK vs US thing?

Was the killer referring to the stuff that melts and turns into water?

(Just let me know if it gets too lame.)

"Morse once solved on a murder on the basis that the killer had left a message using 'ise' instead of 'ize', concluding that he was dealing with someone badly educated"

Yes, it was viewed by the pedantic Dexter/Morse as a barbaric Americanism which would not be used by an English academic. Ironically, the reverse of the current assumption by many would-be English pedants that ize is the barbaric Americanism.

also program vs programme

The z comes from the Latin izo ending, and is therefore sort of more correct in many words. That doesn't make America all correct, of course, as they tend to use the z on some where it hasn't the Latin roots to validate it, as opposed to the Brits who do the converse. I don't care, myself.