It's against Discord community guidelines[1], but their enforcement is terrible. If someone ever bothered to report you for saying that, there's a fair chance you could get banned.

[1] https://discord.com/guidelines says, pretty specifically, "Do not threaten to harm another individual or group of people" and "Do not organize, promote, or support violent extremism. This also includes glorifying violent events or the perpetrators of violent acts". Endorsing killing slaveowners falls under both of those. Here on Hexbear the CoC is worded so doing so is not a violation.