Executor Must Renounce to contest the will
It is fairly common in estate disputes that the executor of the will is also dissatisfied with the same will that the executor is by office bound to carry out and enforce. Since in estate claims it is necessary to sue all parties who have an interest in the estate, it then follows that the executor must be named as a defendant in any estate proceeding, and it is a rule of law that an executor cannot also be a plaintiff in his or her personal capacity. This follows from the legal concept that a party cannot be a plaintiff and the defendant in the same court action – one cannot sue oneself.
Further authority for this is Harrison v Harrison ( 1982) 12 ETR 246
It is established that a person cannot sue himself, even in a different capacity. In Pub. Trustee v. Guar. Trust Co.,  2 S.C.R. 931, 19 C.P.C. 157, 7 E.T.R. 287, 115 D.L.R. (3d) 513, (sub nom. Guar. Trust Co. of Can. v. Berry Estate) 33 N.R. 271, it was held that this principle does not mean that a writ issued by an executor against himself is a nullity; rather it only means that he cannot recover judgment against himself. The majority of the Supreme Court, per Estey J., held that the writ in that case was a mere irregularity which was capable of being corrected by substitution of the Public Trustee for the executor as plaintiff. But it was agreed by counsel and accepted by the court that the matter could not have proceeded to trial and judgment had the executor remained on record as both plaintiff and defendant.