Simple

Time of death 11:20PM. Severe rash. Swelling of the throat. Anaphylaxis.

No peanut products in the kitchen. Peanut allergies. Scent of peanut oil on his lips confirms.

Slippers have clearly been on his feet since 11:20AM. He was exposed while in his house.

Faint traces of lipstick around his mouth indicate recent exchange of saliva. Source of exposure.

He has a practice epi-pen, but no epi-pen in his kitchen, and no mobile phone or epi-pen among the items he threw from his briefcase. Both removed by someone who knew where he kept them.

The victim was murdered by his lover.