I will blast you...... in this context it could mean.... I will hurt you, kill you... I will hit you... is to cause harm to someone, strike someone with your fists or a weapon... I will yell at u.... where "u" is written slang for "you" and this phrase means to speak loudly at someone. I will rip you off... this means you will steal from someone, take from someone

is it bad to follow that pattern?

By the way, never write 'u' or 'ur'. That's only done by small children and teenagers.