What happened that day was constantly repeating in Weiss' mind, driving him insane. After being dragged down by Solan, he spent ten days to escape from the pursuing Desolate Overlord, but although he succeeded he had to pay a price, and that price was huge. Not only was he seriously hurt, he was also infected by two Withering Flames, burning him alive. He might not be able to survive if he couldn't find a Heaven Rank Mage on time to help him remove the Withering Flames.
Moreover, when fighting Solan, he had no other choice but to use part of his power to suppress the Withering Flames in order to reduce the corrosion speed, or he wouldn't be able to stay alive.
The moment he was dragged down by Solan, he swore that even if he could make it alive, he would definitely kill Solan Monchi!