In British Columbia, a spouse (including a common law spouse) or a child (including an independent adult child) may apply to vary a will under the Wills Variation Act, RSBC 1996, c. 490. If the court finds that the testator has not made adequate provision for his or her spouse or child, the court may make such provision as the court decides is adequate, just and equitable in the circumstances.
Section 5 of the Wills Variation Act says that the court may consider evidence of the testator's reasons for disinheriting (or leaving a relatively small amount) to a spouse or child.
The British Columbia Court of Appeal has said that a parent may disinherit an adult child if the parent has rational and valid reasons for disinheriting the child. In Kelly v. Baker (1996), 15 E.T.R. (2d) 21 (BCCA), for example, Mr. Justice Finch (now CJBC) at paragraph 58 held that a parent could disinherit her adult child “... if there were valid and rational reasons at the time of her death - valid in the sense of being based on fact; rational in the sense that there is a logical connection between the reasons and the act of disinheritance.”
A parent might disinherit a child, or leave that child less than his or her siblings, because the parent and child are estranged. The parent, or the parent's lawyer, might be tempted to put a litany of the child's misdeeds in the will, or simply say that the child has mistreated or failed to show love to the parent.
I don't think expressing the parent's dissatisfaction with the child in the will is a good idea. When the child—who may have his or her own grievances against his or her parent or siblings—gets a copy of the will, and sees such comments, the child will perceive the will and the comments as the final insult. The child, who might otherwise have accepted the will, is likely to feel more aggrieved. He or she may feel the need to sue to prove that the parent was at fault. The comments will make it more likely, rather than less likely, that the child will bring a Wills Variation Act claim, or some other challenge to the will.
If a parent has good reasons, but those reasons might antagonize the child who is left little or nothing in the will, my practice is to draft a memorandum for my client, rather than put the reasons in the will. By setting out the reasons in a separate memorandum, the executor is not required to release the memorandum if no one starts a Wills Variation Act claim. If a child does make a claim, then the executor can disclose the memorandum, and the court may consider the reasons. But if the child does not bring an action, there is no reason to needlessly antagonize the child.