Well, from what I can cobble together from scattered information online (here’s a good post on it), the word for “to be” or “am” in Japanese is “desu.” Sometimes it is shortened as “des” or “dess.”
There is no “th” sound in Japanese, so an “s” sound is used as a substitute when porting loan words. So when they “death” it would be expressed as “des,” and in some places I’ve seen it as “desu,” like in Death Note‘s Japanese title.
So a Japanese person saying “death” as the loan word sounds homophonically like “death” to the American. It may also cause some interesting confusion, like in BabyMetal’s “BabyMetal Death” song. They are playing on the linguistic funniness by essentially saying “We Are BabyMetal” in their native language.