It also has to be a grave matter. As in, you know fully that it is evil and wrong.

There is a wide range of mortal sins. Using God's name maliciously in vain is a mortal sin, but if it "slips out" then it's a venial sin.

Sins against the 6th commandment are always mortal. Having a sinful image in your mind, and fighting it off as soon as it gets there is no sin at all, but dwelling on the sinful image, makes it mortally sinful.

Killing someone in self defense is not a sin, however, killing someone deliberately out of anger or malice would be a mortal sin.

Slandering or libeling against someone would be a mortal sin, depending on if you diliberately spoke about a grave matter or a trivial matter. That depends on the conscience.

All sin is bad, no matter what. We should go by the phrase that Saint Dominic Savio used, "Death but not sin."

Keep in mind we all have natural reason. That's something God gives us at the "age of reason," which varies with the individual. Natural reason tells us which things in our hearts we know are wrong.

I hope that helped somewhat.

 

Does it jusify those who hold all sins committed by the sad, depressed, poor or needy people as venial just because such people usually have no better choice?

It can never justify, only mitigate.