The BBC reports on the difference of opinion between the Commons and the Lords on whether infidelity should be a partial defence for murder (and so make it manslaughter rather than murder, for instance).
What's happened is that the Commons passed this particular bit of legislation a while ago, so it went on to the Lords. The Lords amended it to take out the clauses that prevented infidelity from being a partial defence. The Commons have now put those clauses back. It's a small victory.
In the (short) Lord's debate, the voting was very much on party lines, with all the Lib Dem and Conservative Lords voted for the amendment, as did most of the independents. All but one Labour Lord voted against it.
In the Commons, the vote split slightly differently, with all (present) Labour MPs voting to put the clause back, all Conservative MPs voting to keep it out, and the Lib Dems largely abstaining. David Howarth (Lib Dem MP for Cambridge) proposed an amendment that would have changed the terms but not significantly. The debate in the Commons has Claire Ward (Labour MP for Watford) setting out the reasons this is necessary very clearly.
It will mainly be a symbolic victory if this stays in the final bill: after all, a judge who agrees despite the letter of the law that infidelity should be a defence can always impose a lighter sentence, and many will. The arguments against its inclusion that were made in the Commons and Lords, however, expose an unstated assumption: the claim by the Conservatives and the Lib Dem Lords is that whether infidelity is a partial defence should be up to the jury for that case, but for that to be true requires that there should be a significant number of existing cases - not just a "it might happen" - where infidelity is a legitimate reason for "losing control"1 to the extent of murder.
This is what most of the Lords believed, and the Conservative MPs in the Commons. It may well be an attitude shared by many jurors, but that doesn't make it right. It essentially is preserving an existing expectation that infidelity is (and should be) punishable by death in at least some circumstances: an attitude that we're very quick to condemn in other countries if it's the state rather than the partner (despite Lord Gresford's claim based on the sample of cases he remembers, it is usually the male partner) that carries out the execution.
Let's not pretend either that most of these cases of murder won't have been preceded by some other forms of abuse, including violence. This isn't usually going to be someone who "just snapped", it's going to be someone who has been previously violent. (and because of the victim-blaming culture around abuse and domestic violence, which the [...] are trying to reinforce, and the extents to which abusers conceal their activities, and other people try not to notice them, this fact may only be known by the now dead victim). Jack Straw points out that the existence of a murder charge in the first place implies intent to cause serious harm.
1 Now, everyone handles anger differently, but I think that if someone gets angry enough to kill or seriously injure someone, in a situation that was not before their violence a threat to anyone's life or safety, then future public safety is best served by their incarceration - one of the few cases where imprisonment is necessary. "losing control" implies that they had as much control over their actions as someone who skids their car on an invisible patch of ice and runs into a pedestrian before they can stop, and is not an appropriate phrase.