Todd and Stacey met at a young age and quickly found themselves with 3 children under 2 years of age.  This would be hard for many people and could cause a significant amount of stress.  Was it too much stress to cause Todd to snap and murder his children?  

Sources:

https://murderpedia.org/male.W/w1/willingham-cameron.htm

https://deathpenaltyinfo.org/news/former-prosecutor-on-trial-on-charges-that-his-misconduct-led-to-wrongful-execution-of-cameron-willingham

https://en.wikipedia.org/wiki/Cameron_Todd_Willingham

https://www.theguardian.com/us-news/2015/mar/19/texas-prosecutor-accused-misconduct-execution-case

https://www.texastribune.org/2014/04/03/citing-new-evidence-innocence-project-calls-pardon/

https://www.pbs.org/wgbh/pages/frontline/death-by-fire/

Investigation Discovery: Evil Lives Here Season 9 Episode 1