अर्जुन
In plain words
Arjuna said: Then by what is a person impelled to commit sin, even unwillingly, Varshneya, as though driven by force?
Word for word
Then impelled by what does this person commit sin, even not wishing [to], O Vārṣṇeya, as if compelled by force?
The commentators
Bhagavadgītābhāṣyam· bhāṣya
अथ केन हेतुभूतेन प्रयुक्तः सन् राज्ञेव भृत्यः अयं पापं कर्म चरति आचरति पूरुषः पुरुषः स्वयम् अनिच्छन् अपि हे वार्ष्णेय वृष्णिकुलप्रसूत, बलात् इव नियोजितः राज्ञेव इत्युक्तो दृष्टान्तः ॥
In plain words
So, Kṛṣṇa, descendant of Vṛṣṇi — what is it that drives a man, what actual cause is at work, when he commits a wrong act like a servant ordered about by his king, even though he himself doesn't want to? It's as if he were being forced into it, the way a servant is forced by his master. That's the picture Arjuna is drawing, before he asks his real question.
Close to the Sanskrit
Now, impelled by what, serving as a cause, being so, like a servant by a king, does this man, this person, commit, perform, sinful action, himself, even though unwilling, O descendant of Vṛṣṇi, born in the Vṛṣṇi clan — as if compelled by force, like [a servant] by a king — thus is stated the illustration.