Can't figure out the conjugation of the verb 'matar'
 

Hola,

So I'm still learning spanish, and I was wondering why we'd say '(yo) maté' when we want to say 'I killed', but we should say 'mató' as in 'It was me who killed him': 'Fui yo quien lo mató'. I mean it's still ME who killed HIM, so shouldn't I say 'maté' instead? That would make more sense in my opinion... I'm pretty sure I'm wrong, since I've heard this (the use of 'mató' instead of 'maté') many times in several movies. Could somebody please help me out here and explain to me why it should be this way?

Gracias!

Because "quien" introduces a third person.

Yo lo maté. -> I killed him.
Fui yo quien lo mató. -> I was the one who killed him.

Ahhh, ahora lo entiendo. Entonces, puedo decir 'Fui yo que lo maté' o 'Fui yo quien lo mató', no?

The explanation is because after a relative we use 3rd person sigular like in English.
Yo maté
Tu mataste
el mató
I,m not completely sure but if you think about it , its quite logical.

Muchas gracias, ahora lo entiendo. I was really confused because of that 3rd person-thing, but know I totally get it and it's indeed quite logical. Muy estúpido de mi parte...

I was him (fui yo) + he killed him (lo mató) - the first person in the 1st part is clarifying the 3rd person in the 2nd part :)

I believe the quien is simply joining the two parts together just like in English.

I was the one who killed him
Fui yo quien lo mató

Could you also say "it was me who killed him"? Any difference? Less formal or something like that?

Yes, they are basically equivalent, and they are both variations on "I killed him".

Expanding a basic sentence such as "I killed him" to either "I was the one who killed him" or "It was me who killed him" typically happens in a conversation where the facts are relayed over several sentences. The expanded sentences typically present the new, contrasting or emphasized information (I am responsible) and relate it to previously-known information (someone killed him).

Thanks. In respect to the explanation, it is the same in Spanish.