Some fans are devastated. Levi has a huge fan following, and his death in a fanfic can be like losing a real friend for them. They might stop reading the fanfic altogether if they can't bear the thought of him being dead.
Eren would probably be very curious. He's always been eager to learn and understand new things. Meeting kids from another time would be fascinating for him. He might ask them a lot of questions about their world and how it differs from his. Levi, on the other hand, would be more guarded. He'd be suspicious at first, but once he realizes they're not a threat, he might start to warm up and show his more protective side towards the kids.
It's hard to say. 'Levi x Eren' fanfics are quite common in certain fandoms, but the 'Levi is a criminal' aspect makes it a more specific and perhaps niche sub - genre. If there are a lot of fans who like to see dark or morally ambiguous takes on the characters, it could potentially be popular within that subset of the fandom. However, it also might not be mainstream popular as it goes against the more typical heroic portrayals of the characters.
In the 'Attack on Titan' series, there is no clearly defined canon romantic interest for Levi. His character is mainly focused on his role as a soldier and his fight against the Titans. However, many fans like to ship him with various characters in fan - made love stories.
Maybe Levi is a criminal in the sense that he's part of an underground rebellion against an unjust regime. Eren, who is initially on the side of the establishment, meets Levi and gradually comes to understand his motives. Their relationship could develop as Eren has to decide whether to side with Levi or stay loyal to the regime, all while a romantic connection forms between them.