it was classified as a deadly nightshade

It's worth mentioning that while this seems insane to us now, it probably made a ton of sense at the time. The key is that tomatoes are in the same botanical family, so the plants have a lot in common. For example, here's what black nightshade fruit looks like:

You might see something that and think it resembles a dark tomato. Hundreds of years ago, people looked at a tomato and saw a red nightshade.