"Tener mala leche" = To have bad milk*

To be ill-intentioned, to be mean.

Mira lo que ha dicho, es que tiene muy mala leche.
Ten cuidado con él porque tinene mala leche y sus intenciones no son buenas...
Tiene cara de mala leche.

NOTE: do not confuse "tener mala leche" (bad intentions) with "tener malas pulgas" (bad temper) as they are not synonym expressions. However! "estar de mala leche" y "estar de malas pulgas" are synonym expressions meaning both "to be angry or in a bad mood". I know! This is confusing, but you can do it!!!