It was an unexpected message according to Jenny Madestam, political scientist at Södertörn University.
This is not something that has leaked out via the media, that such thoughts existed, so I think many were surprised, she says to TT.
Even Marja Lemne, Social Democrat-connected political scientist at Södertörn University, was surprised.
It's a mystery right now. It's a bit strange how this resignation comes.
Possible Conflict
Similar messages are usually announced during a press conference with a new, ready-made successor, according to Lemne.
The way he went out with it himself, via X, suggests that there may be conflicts behind, even if it's denied.
According to Jenny Madestam, it's very possible that Billström is simply generally exhausted or has grown tired.
But if you leave in the middle of a term and are a political animal like Billström, then it's not too wild a speculation that it's not just about wanting to do something else in life, she says.
Unclear Future
What Billström will do now is still unknown.
He may have an offer from the business world and now it's the cooling-off period that applies when you switch, says Lemne.
It's common for top politicians to "crown their political career" with heavy assignments as director-general or ambassador, according to Jenny Madestam.
It also depends on what caused this. If it's conflicts and if they're with the Prime Minister, maybe it will have consequences for what he'll do next.
Immediately after the resignation message, speculations began about who could take over the ministerial post. Lemne mentions Migration Minister Maria Malmer Stenergard (M) and Defense Minister Pål Jonson (M).
You should probably look around among the other ministers, she says.
Another name is Henrik Landerholm, national security adviser to the government.
He's an experienced politician and extremely knowledgeable about security policy issues. Moreover, he's well-known to be close to Ulf Kristersson, says Madestam.