Reading from John 5:14
Afterward Jesus findeth him in the temple, and said unto him, Behold, thou art made whole: sin no more, lest a worse thing come unto thee. Does it mean all those whose sins are forgiven and keep on sinning will have more penalty?

Simple. "Sin no more" means, stop breaking God's commandments (1 John 3:4).

And "lest a worse thing come upon you" depicts the wrath of the enemy (Satan), having been rescued before by a superior authority (God), and then falling again through sin into his grip.

Clear now

zico530:
Reading from John 5:14
Afterward Jesus findeth him in the temple, and said unto him, Behold, thou art made whole: sin no more, lest a worse thing come unto thee. Does it mean all those whose sins are forgiven and keep on sinning will have more penalty?

⁸Jesus told him, “Stand up, pick up your mat, and walk!”
⁹Instantly, the man was healed! He rolled up his sleeping mat and began walking! But this miracle happened on the Sabbath,
¹⁰so the Jewish leaders objected. They said to the man who was cured, “You can’t work on the Sabbath! The law doesn’t allow you to carry that sleeping mat!”
¹¹But he replied, “The man who healed me told me, ‘Pick up your mat and walk.’”
¹²“Who said such a thing as that?” they demanded.
¹³The man didn’t know, for Jesus had disappeared into the crowd.
¹⁴But afterward Jesus found him in the Temple and told him,
“Now you are well; so stop sinning, or something even worse may happen to you.”
- John 5:8-14

No, it doesn't in that sense, mean all those whose sins are forgiven and keep on sinning will have more penalty

It means, a particular habit, lifestyle, routine, practice, weakness, self-gratification etcetera, which was bad before becoming healed, sound or healthy again, could become something more serious or more unpleasant, if the particular habit, lifestyle, routine, practice, weakness, self-gratification etcetera is relaunched or reintroduced

As a matter of fact and truth, the study of the word ''sin'' or ''sinning'' actually means ''missing the target mark''
It can mean ''missing the target mark'' as regards with God, fellow human being or with the right use of any other object
In John 5:14, it is used as a verb (i.e. as in, ''sin no more'' means ''stop doing'' the thing wrong'')

The healed crippled in John 5:14 and the woman caught in adultery rescued from getting stoned to death in John 8:11,
are the only two recorded people in the Bible Jesus tells to ''sin no more'' (i.e. stop the ''missing the target mark'')

John 5:14 had nothing to do with penalties or punishments but more to do with cause and effect
It's has to do with a combination of action and reaction(s)

John 5:14 is saying, after being made whole, the healed crippled man was warned not to ''sin'' aka ''miss the target mark'' to avoid the risk of a worse illness befalling or be visiting on him

For example, smoking causes the following effects:
infertility, stained or bad teeth, heart disease, coughing, anxiety & irritability, lung cancer, problems with pregnancy, problems for newborns, erectile dysfunction, blood cancer etcetera

Note, not all the effects in the above happens at the same time, but as the smoking habit continues or increases, other worse ailments or illness come unto the avid smoker.
It's bad enough having bad breath or coughing from being a smoker
but imagine worse things like cancer etcetera happening if the smoking habit persists

Am surprised you didnt find the pastors Jewish leaders' perplexity of Jesus' command that the man should pick up his mat and walk
and their ''You can’t work on the Sabbath! The law doesn’t allow you to carry that sleeping mat!'' baffled remark
inside the John 5:8-13 build up to John 5:14 intriguing