Collusion between a small number of players to control prices in a market is called a “cartel” and it’s a significant departure from the concept of a “free market”.
Cartels happen in markets that are not free, because in a free market that price fixing would lead to insulin sales being so profitable that new manufacturers would get into the game and the competition would bring prices back down to their normal levels.
One can argue whichever way they like for the overall benefit of the tight regulations we have on things like insulin production and distribution, but it is a fact that one effect of that tight regulation is extremely high barriers to entry, and hence the formation of price cartels such as we see now.