You're correct to suggest that the known killings that Holmes was involved with were apparently associated either with insurance fraud or problems associated with breach of promise, but the FBI definition of the term "serial killer" simply refers to a number of consecutive murders committed by the same person(s) with an interval of time between them; it does not in any way exclude this sort of murder.

Anyway – there is always more to say, but it's certainly worth mentioning that Holmes is an historically problematic example of a "serial killer" in any case. The usual accounts given of him, including the famous one offered by Erik Larson in his Devil in the White City, are not supported by even half-decent evidence that he killed more than five people, a very long way from the "200" often attributed to his activities. You might like to go over my earlier review of the case while you wait for fresh answers to your question:

Was H.H. Holmes as prolific a serial killer as he claimed to be, or were his exploits largely exaggerated by himself and the media?