That, and the fact that Free Actions aren't a dime a dozen. Eight Free Actions in a Turn? No way.
So eight swings with the force swords is alright but a free action at the end of every sword hit is not? Not sure how that makes sense.
WS 60, PR 6, Lightning Attack, Two-Weapon Wielder, and dual-wielding Force weapons = 13 potential hits, maximum. Warp Time adds PR to WS as an Unnatural Characteristic Bonus. Only one Lightning Attack may be made, the secondary weapon may strike once. Warp Time descibes the Psyker's/Sorcerer's movements as being of "terrifying speed", so yes, 8 hits (potential Damage to Horde Magnitude) in one Turn is fine.
DoS - Degrees of Success
DoF = Degree of Failure
DoD = Degrees of Difference
Page 234, Free Actions, first sentence: "A Free Action takes only a moment and requires no real effort by the character." Further: "…and there is no formal limit to the number of Free Actions one character can take. The GM should use common sense to set resonable limits… Examples of Free Actions include dropping a weapon or speaking a few words." Reasonable limits. Page 130, Quick Draw (Talent), allows a character to "…draw and Ready a weapon as a Free Action…" A weapon. Singular.
Additionally, as the act of channeling psychic force and killing will is represented as the Psyker/Sorcerer making a Focus Power Test using Opposed Willpower, I would rule that at least some effort is required. To validate this position:
There is no indication in the rules for weapons with the Force Quality (pg 149) regarding the modualtion (Fettered, Unfettered, Push) of the channeling of psychic force and killing will, other than the rules detailing the amount of Damage caused by weapons with the Force Quality: "For every point of Psy Rating the wielder has the weapon's Damage and Penetration increase by +1." This implies the unmodified PR of the wielder, which translates as Unfettered. The Psyker/Sorcerer has no choice in the matter; this Opposed Focus Power Test is Unfettered. Further to this, pg 208, FOCUS POWER TEST, details the rules as follows: "Many powers also indicate an Opposed Test… Note that even if the psyker gathers fewer DoS than his opponent and does not activate the power, he still can generate a disturbance in the warp (Phychic Phenomena/Perils of the Warp) if using the power at Unfettered or Push level."
A reasonable interpretation would be to consider the act of channeling psychic force and killing will as, at the very least, having an Action Subtype of Concentration. As a GM, I would also rule it has the Action Subtype of Attack, my reasoning being simple and as follows: In the event it becomes possible that any other Psychic Power that would cause Damage to a Horde's Magnitude could be activated by a Psyker/Sorcerer who is also intent on channeling killing will in the same Turn, ascribing the Action Subtypes of Attack and Concentration will prevent such an unreasonable use of Actions.
Allowing for more than one Free Action when channeling killing will means each Free Action Opposed Focus Power Test could then generate Phenomena/Perils, and that would be a lot of unchecked warp energy for "no real effort".
I will add a fifth bullet point to the order from my previous post, and clarify as follows:
Psyker/Sorcerer calculates the number of hits caused by his/her attack
Psyker/Sorcerer rolls Damage for any hits
If at least one hit from a weapon with the Force Quality causes Damage (defeats AP and TB) then he/she may channel psychic force and killing will by making a single (Free Action) Focus Power Test using Opposed Willpower
For each DoS achieved on the (Free Action) Focus Power Test (clarified as the Psyker/Sorcerer succeeding on the Test and achieving more DoS than his opponent) the Psyker/Sorcerer may increase the Damage caused to Horde Magnitude by +1
Regardless of the number of hits from weapons possessing the Force Quality that are wielded by a Psyker/Sorcerer, he/she may only ever make a single (Free Action) Focus Power Test using Opposed Willpower in his/her Turn
A Horde of Imperial Guardsmen, WP 25, fails their Opposed Willpower Test by 3 DoF, while the Psyker/Sorcerer achieves 4 DoS. The total DoD is 7, so the Horde takes an additional 7 points of Damage to its Magnitude. Adding this to 8, we have a total of 15 points of Damage to the Horde Magnitude. The amount of extra Damage caused to a Horde's Magnitude with killing will (if any) is completely random, calculated from DoS, DoF, and DoD. It could very well be zero.
Each Psychic Power beyond the first that is Sustained increases the chances of Perils by +10. A Heavy Flamer can do 8+d5 hits to a Horde. Based on these factors alone, as a full-time GM I consider all of the above to be within reasonable limits.