I searched for this but no source seems to offer any clarification as to why he was killed. His killer also doesn't seem to have been punished in any way. This is just very strange to me. Had this something to do with the anymosity between Danes and Swedes?
From his Wikipedia page:
" He designed the exquisite Riddarhuset but was killed by the nobleman Erik Oxenstierna (1624-1656), a week after construction had begun. "
Translation from swedish Wikipedia (google translate so correct me if it's wrong), again offering no further clarification or a motive:
"Simon de la Vallée was killed in Stockholm in 1642. The architect was knocked over on Stortorget by Erik Carlsson Oxenstierna, who also hit him with the gun barrel so hard that he later died. "
Worth noting about the killer, Erik Karlsson (Oxenstierna), is that he was a part of the Oxenstierna family that at the time was the most powerful family in Sweden second only to the king; and frankly at times above the king. Axel Gustafsson (Oxenstierna) was Rikskansler (Lord High Chancellor, basically Prime Minister) between 1612 and 1654 and Gabriel Gustafsson (Oxenstierna) was Riksdrots (Lord High Seneschal, basically Minister of Justice) between 1634 and 1640, and Gabriel Bengtsson (Oxenstierna) was Riksskattmästare (Lord High Treasurer, basically Minister of Finance) between 1634 and 1652. I do not find exactly how Erik Karlsson (Oxenstierna) is related to the others, but according to the National Archive's page on Simon de la Vallée so did de la Vallée work for both Erik Karlsson (Oxenstierna) and Gabriel Bengtsson (Oxenstierna) so he was clearly connected to the people within the family who ruled the country at the time^(1).
It is also, based on the same National Archive source, worth noting that de la Vallée had been employed by Erik Karlsson (Oxenstierna) on a few projects before the murder, so they did know each other.
Regarding the fight per se does it seem all sources just have it down as a verbal disagreement, for example a classic biographical lexicon has it as "brutal verbal disagreement [...] Oxenstierna took his sword and pierced De la V." (my translation)^(2). So if he got ridden down, hit with a gun or stabbed seems unclear and there seems to be no information about the content of the disagreement. There is some information though regarding the court process, I could unfortunately not find any proper source for it but Ridarhuset itself has written about it on social media; in short, Erik Karlsson (Oxenstierna) ended up paying de la Vallée's widow and children but was protected from serious consequences^(3).
After digging through the question for a while do I not find that it would have any connection to any conflict with Denmark or the then ongoing Thirty Years' War or the upcoming Torstenson War. It is rather the story of a young nobleman with a bad temper who cut down an architect but who can't be touched due to his family.
1 https://sok.riksarkivet.se/sbl/mobil/Artikel/17433
2 http://runeberg.org/sbh/a0237.html
3 https://www.facebook.com/riddarhuset/posts/1367660196678232/