Scythe deals with death, murder, and moral philosophy but is not gratuitously violent. It is published as YA and commonly read by ages 13+. The content gets progressively darker in Books 2 and 3. The premise (licensed killing) requires emotional maturity.
Yes — the trilogy is complete: Scythe (2016), Thunderhead (2018), and The Toll (2019). All three books are published.
Yes — the trilogy should be read in order. Each book builds directly on the previous and the story is one continuous narrative.