「んでいます」 is not a single unit, you're splitting the word in the wrong place.
How to break down 死んでいます
「死んで」 is the -te form of 「死ぬ」. You then connect that to 「いる」 (or if you're being polite, 「います」). Together, they form the -te-iru form of 「死ぬ」: 「死んでいる」
The -te-iru form
The -te-iru form of a verb has multiple meanings, and depending on the type of verb, those meanings differ. In the case of 「死ぬ」 (which is a state-change verb)...
"is dead" (stative reading)
"are dying" (iterative meaning) (like, multiple different things are dying)
So, 「瀬間は死んでいます。」 unambiguously means "瀬間 is dead.", given the understanding that 「瀬間」 is a person's name (preventing the iterative meaning, since that would be a singular subject). On the other hand, if 「瀬間」 were the name of a species or something, then it'd likely be the iterative meaning.
What you said
You said 「瀬間は死でした。」, which means something like "瀬間 was death." In your sentence, 「でした」 is the polite past-tense copula, and 「死」 is the noun for "death". I can't think of any time you'd want to say this outside of fiction. That said, your sentence is at least grammatical.