As a young boy, Leonard had walked in on his mother, Maria, sleeping with a man who was not his father while Carl was abroad. When their house was stormed, Maria was able to hide Tessa away, but thrust Leonard to the enemy. The children’s genius was well known at this point, and she likely hoped that giving him to them would get them to spare her, and Tessa. It’s likely she did this so willingly in an attempt to preserve her lifestyle, simply because Leonard knew her secret. They killed her anyways, and the two kids would have been kidnapped if Borda had not arrived after being contacted by Carl and saved them.