Sweden uses a party list proportional representation system. Seats are allocated based on how many votes a party gets, and they are allocated to parties first. The selection of who then sits in those seats then comes down to ”personröster” (literally “person-votes”, but I think they're conventionally translated as “preferential votes” or something like that), which are optional. Most voters do not put a cross next to their preferred candidate, they just put the party ballot paper in the envelope and the envelope goes in the box. And because they're optional, there's therefore a threshold for how many personröster a candidate has to receive to be elected. That threshold is 5% of voters in the particular electoral district. Only candidates that get more than 5% of votes are even considered in this process; if there's none (or not enough), then the candidates from the party list are taken.

The seat would have gone to the Left Party either way. But only this guy specifically got enough personal support to get elected. There's nothing that crazy about it. Participative democracy rewards those who choose to participate. In this case, it's a two-tier participation system. People who vote for a party, but not for a specific candidate, are letting the people who do vote for a specific candidate choose for them. And that's fine.