Well of you provide a service and it's knowingly used by criminals and you just implement features to benefit the criminal activity but make no effort to curb it. Yes, your an accomplice.
I like Telegram, a lot, but if you're ignoring the fact that large parts of it openly function as a mall for criminal services (and it's 100x easier to find that stuff than via Tor, for example) you're not being honest with yourself. A lot of people here are just reflexively assuming its mean cops vs encryption because that's an issue tehy personally care about, and ignoring any other context.