What is the ruling on one who washes his feet after performing ablution (wudu') due to the splashing of contaminated water from the bathroom floor, resulting from family members using toilet slippers stained with urine? Is his action considered correct or a form of obsessive-compulsive disorder (waswas)?

The default state of shoes used in the restroom is purity, and they are not judged to be impure, nor is anything they touch judged impure, merely based on doubt. Therefore, washing the feet after ablution based on that is incorrect and may be a form of waswasa (obsessive whispering/doubt), which is a dangerous ailment that should be shunned.