The will shall be disallowed in any of the following cases:
(1) If the formalities required by law have not been complied with;
(2) If the testator was insane, or otherwise mentally incapable of making a will, at the time of its execution;
(3) If it was executed through force or under duress, or the influence of fear, or threats;
(4) If it was procured by undue and improper pressure and influence, on the part of the beneficiary or of some other person;
(5) If the signature of the testator was procured by fraud;
(6) If the testator acted by mistake or did not intend that the instrument he signed should be his will at the time of affixing his signature thereto. (n)