rk's weakness is his past, that is the main problem. the pain he got in the past made him lose faith in love forever and when suddenly he got love again he wanted to be sure before he accepted her.
according to my POV he never doubted madhu's love or her innocence. when dips tried to put poison in rk ear about madhu, rk grabbed her by her throat and said madhu is nothing like that. he even tested dips once remember in the hospital where she said she can slit her wrist to prove she loves him right in front of madhu so if that was ok then what is wrong if he tested madhu for madhu's sake?it is not himself who wanted to test madhu's love for him but he wanted the world to see whatever they have between them is love and nothing but love. so many things has happened between them that it was kind of hard to make them confess so the cvs took this way.whatever he did was to shut dip up once an for all with her 4 phere ki biwi and he won't ever get love, he wanted to prove that he will get love and madhu is his love so she is wrong. trying to prove dips wrong he harmed himself along with his wife. he thought this game is going to be between him and dips but madhu found out, he didn't wanted to hurt her but she still got hurt.