All good questions, gregroz. Here the neuter singular adjective κακόν ("evil") is treated as a noun ("an evil thing"). You can do that with almost any neuter adjective (καλόν "a beautiful thing," αἰδοῖον "a shameful thing," πλήσιον "a close thing," etc.). Suspicion is what kind of evil? A terrible evil! Suspicion is a terrible evil for people/humans.
Sophocles famously does this with the neuter plural adjective δεινά in the Antigone, verse 332: πολλά τὰ δεινά — "many are the terrible (things)."