Death Siphon - Low level DK

[Tumzie]

Dear players,

My bot uses Obliterate and Frost Strike as his main skills to kill mobs, but they deal 700 dmg whereas Death Siphon deals over 4k.
How to I tell my bot to use Death Siphon whenever its useable?

Thanks in advance, would increase killspeed by over 600%.

Yours faithfully,

Tumzie