Here's more context Jesus was condemning those who keep their own rules over obeying the commandments of God quoting directly from the Ten Commandments
Mat 15: 3 He answered and said to them, “Why do you also transgress the commandment of God because of your tradition? 4 For God commanded, saying, ‘Honor your father and your mother’; and, ‘He who curses father or mother, let him be put to death.’ 5 But you say, ‘Whoever says to his father or mother, “Whatever profit you might have received from me is a gift to God”— 6 then he need not honor his father [a]or mother.’ Thus you have made the [b]commandment of God of no effect by your tradition. 7 Hypocrites! Well did Isaiah prophesy about you, saying:
8 ‘These people [c]draw near to Me with their mouth,
And honor Me with their lips,
But their heart is far from Me.
9 And in vain they worship Me,
Teaching as doctrines the commandments of men.’ ”
Which goes with
And then I will declare to them, ‘I never knew you; depart from Me, you who practice lawlessness!’
1 John 2:3 Now by this we know that we know Him, if we keep His commandments. 4 He who says, “I know Him,” and does not keep His commandments, is a liar, and the truth is not in him.
Mat 15: 3 He answered and said to them, “Why do you also transgress the commandment of God because of your tradition? 4 For God commanded, saying, ‘Honor your father and your mother’; and, ‘He who curses father or mother, let him be put to death.’ 5 But you say, ‘Whoever says to his father or mother, “Whatever profit you might have received from me is a gift to God”— 6 then he need not honor his father [a]or mother.’ Thus you have made the [b]commandment of God of no effect by your tradition. 7 Hypocrites! Well did Isaiah prophesy about you, saying:
8 ‘These people [c]draw near to Me with their mouth,
And honor Me with their lips,
But their heart is far from Me.
9 And in vain they worship Me,
Teaching as doctrines the commandments of men.’ ”
Which goes with
And then I will declare to them, ‘I never knew you; depart from Me, you who practice lawlessness!’
1 John 2:3 Now by this we know that we know Him, if we keep His commandments. 4 He who says, “I know Him,” and does not keep His commandments, is a liar, and the truth is not in him.
Yeah in the OT, it was a serious offense and death was to be carried out. They practiced lawlessness by not killing, the old testament didn't play around.
Matt 15:4 For God commanded, saying, ‘Honor your father and your mother’; and, ‘He who curses father or mother, let him be put to death.’