Stennis does not have a strong support of his countrymen. That's why he decided to aid Saskia the Dragonslayer in the fight against Henselt troops to reassert their right to the throne. Stennis is supported only the nobles and ordinary peasants hate him.
He's so afraid of losing power to Saskia, so he tries to poison her and eliminate her as his political opponent.An angry mob burdens him with the blame for poisoning Saskia. Geralt of Rivia has 2 options:
To allows the angry mob the lynching, letting Stennis be brutally murdered.
To protect him from angry mob. That allows Stennis to become 13th king of Aedirn. After taking the throne he tries to control the chaos reigning in Aedirn.