It's a special meaning of hacer.
DRAE:
- tr. Obligar a que se ejecute aquello que expresa un verbo en infinitivo o una cláusula introducida por que. Le hizo venir. Hizo que nos fuésemos.
Lexico:
El verbo hacer + infinitivo + complemento suele tener valor de voz pasiva cuando no se expresa el sujeto de la acción que el infinitivo señala. En estos casos se traduce por to have + object + participle:
- hizo arreglar el coche = he had his car repaired
- hizo construir la casa = he had the house built
Note that in the last example, if the listener knows that we're talking about the house, then we can use a pronoun, and the sentence becomes "La hizo construir" (I had it built).
So, "Lo hicimos matar" means "We had him killed."
The weird thing about this definition is that it actually encompasses two slightly different patterns. In "Le hizo venir," we're describing an action carried out by the referent of the object pronoun, and in "La hizo construir" and "Lo hicimos matar," we're describing an action carried out on the referent of the object pronoun.
In the case of "Le hizo venir," notice that the object pronoun is indirect. In the other examples, the object pronoun is direct.
In both cases, the subject made the action happen.