This is a matter of public policy – clearly if you could still inherit after killing someone, there would be a rather large motive for doing so!
The Court has the power to modify this rule if it is satisfied that the circumstances of the case justify doing so, provided that the beneficiary has not been convicted of murder. In deciding whether to modify the rule, the Court will have regard to the conduct of both the beneficiary and the Deceased, and any other circumstances that the Court regards as material. (Section 2(2) Forfeiture Act 1982).
A clear example of the application of the forfeiture rule can be found in Cleaver v Mutual Rescue Fund Life Association  1 QB 147. In that case, the husband took out an insurance policy on his life worth £2,000 (a princely sum in 1892) for the benefit of his wife. When he later died as a result of poison administered by his less-than-loving spouse, the Court unsurprisingly applied the forfeiture rule and said the wife could not have the insurance money.
Another case which demonstrates how the rule is applied, is Re Crippen’s Estate  P 108. In this case, the Probate Court allowed one of the Deceased’s next of kin to administer her estate, skipping over the husband who had been found guilty of her murder.
Although these cases are very obvious examples of how the Act is applied, the forfeiture rule does not apply solely to murder cases. It has over time been applied where the case involves manslaughter or a suicide pact.
On 1 February 2012 the Estates of Deceased Persons (Forfeiture Rule and Law of Succession) Act 2011 came into force, clarifying the law in relation to determining who may inherit a beneficiary’s interest that is forfeited under the forfeiture rule. So, for example, if the victim died intestate, the person who has forfeited their interest as a result of their crimes is treated as having died before the Deceased, and the next person entitled in order of priority per section 46 of the Administration of Estates Act 1925. Clause 2 of the 2011 Act sets out what happens if the Deceased left a Will – the clause is analogous with what happens on intestacy. Note that the Act does not alter the Court’s discretion to waive forfeiture where it regards such a waiver to be in the interests of justice.
Note that the result of a 2006 case, Land v Land*, is that claimants can now choose between claiming for relief from forfeiture under Section 2 of the Forfeiture Act 1982, or claiming for reasonable financial provision under the Inheritance (Provision for Families and Independents) Act 1975. Both options are subject to very strict time limits. If the option is there to claim relief under the Forfeiture Act, this will often be the better choice because they may end up with the whole of the Deceased’s estate – whereas under the Inheritance Act, the award by the Court will be for reasonable financial provision. The two factors to consider when deciding how to claim will be (i) what the claimant is likely to receive under each avenue and (ii) the time limits: three months from conviction for a relief from forfeiture claim, or six months from issue of grant of probate for an Inheritance Act 1975 claim.
* EWHC 2069 (Ch),  1 All ER 324