In Jain Dharma, knowingly killing a panchendriya jīv binds very grave pāpa karma. In simple words, it binds the karma of prāṇātipāt / hiṃsā very strongly, with intense asrava and bandha due to cruel intention and strong passions. The exact bondage depends on the bhāva behind the act, but it is considered one of the most serious sins, especially when done deliberately.

Generally, such an act can strengthen bondage of:

• pāpa prakṛti related to violence,
• moha and other deluding karmas,
• and, depending on the nature of the being harmed and the inner state, very severe future suffering.

For the second part:

2) What happens when you do prāyaścitta for that sin?

In Jainism, prāyaścitta means sincere repentance and purification. It does not magically erase the already-bound karma at once, but it helps in these ways:

• stops new sinful bondage,
• weakens the force of the past karma,
• purifies the soul through remorse, confession, fasting, repentance, restraint, and atonement,
• and helps reduce future suffering by turning the mind away from violence.

The most important part is true inner repentance:

• feeling deep regret,
• promising never to repeat it,
• adopting ahiṃsā more strictly,
• and following the proper austerity or penance given by a sadhu/sādhvī or knowledgeable Jain guide.

So, in short:

• Knowingly killing a panchendriya jīv binds very heavy pāpa karma.
• Prāyaścitta does not wipe it away instantly, but it purifies, reduces its intensity, and prevents further bondage when done sincerely.