In plain words
These worlds would fall into ruin if I did not act; I would become the cause of intermixture and would destroy these beings.
Word for word
These worlds would perish if I did not perform action; and I would be the maker of confusion, [and] would destroy these beings.
The commentators
Bhagavadgītābhāṣyam· bhāṣya
उत्सीदेयुः विनश्येयुः इमे सर्वे लोकाः लोकस्थितिनिमित्तस्य कर्मणः अभावात् न कुर्यां कर्म चेत् अहम् । किञ्च, सङ्करस्य च कर्ता स्याम् । तेन कारणेन उपहन्याम् इमाः प्रजाः । प्रजानामनुग्रहाय प्रवृत्तः उपहतिम् उपहननं कुर्याम् इत्यर्थः । मम ईश्वरस्य अननुरूपमापद्येत ॥ २४ ॥
In plain words
"They would sink down" — meaning they would simply perish, all these worlds, because the action that keeps the world standing would no longer be there, if I stopped acting. And there is more: I would be the author of saṅkara, the collapse of every distinction into a blur. For that reason I would be the ruin of these creatures. The one who took up action for the welfare of beings would end up destroying them — that is the sense. And that would be entirely unbecoming of me, who am the Lord.
Close to the Sanskrit
"They would sink down" — they would perish — all these worlds, from the absence of action which is the cause of the world's continuance, if I were not to perform action. Moreover, I would also be the author of confusion. For that reason I would strike down these creatures: engaged in activity for the favour of creatures, I would inflict injury — that is, destruction — upon them; this is the meaning. It would turn out to be unbefitting me, the Lord.