One of the easiest things to do wrong on a homemade will (one done through a kit) is in the signing. Signing your own name at the bottom is not the problem; getting witnesses is.
I was recently asked if a will was valid based on the fact that one of the beneficiaries had witnessed it. That situation is one of the most distressing for a beneficiary, because yes, the will is valid, but the gift completely fails. If you witness a will that you’re named in, you are going to get nothing. It’s a failsafe to ensure that you aren’t forcing someone to sign a will naming you as the beneficiary when there has to at least be someone else there to witness. It’s also a reason why having a lawyer help with your will makes your will that much better.